I'm in the middle of rebuilding my bench and need a bit of advice.

I have an 8 ft bench that's a torsion design with all 2x6 construction and a 1.5" plywood top - it's solid as a rock.

I've been using Inline Fab's Quick Change mounting system and it's a perfect solution for those of us with a single bench.

However, I have added a 2nd Dillon 650, which I don't plan to take on/off like my turret and single stage presses.

My goal is to have a separate/dedicated 650s for both small and large primer calibers.

I want to incorporate flexibility into my bench mounting - essentially, I want to move the 650s to the end of the bench when I'm not using them, but without taking them completely off to be stored elsewhere.

I've looked around quite a bit and it seems that other reloaders either utilize a T Track system or threaded T nuts to fasten presses to the bench in various places.

My idea is to utilize the Inline Fab mounts, but simply move them where I want them with a track system or permanent T nut locations (countersunk into the bench top).

Some of the T Track systems I've seen on YouTube seem to have quite a bit of movement at the bottom of a press stroke, especially when sizing larger brass. I do not want to introduce any movement to my current system.

For those of you who use a track system, do you sacrifice any of the natural rigidity in your bench?

The other solution is using the threaded T nuts that are sunk into the bench top. However, some say it's possible for them to loosen over time. That would present a new set of problems if accurate.

Does anyone have any experience with using threaded nuts?

These are the models that have been suggested to me: Amazon Threaded T Nuts

Finally, is there another method that I haven't mentioned? I have considered drilling all the way through the bench and using a bolt/nut solution, but don't really want to drill through the 7 inches of plywood/2x6s on the front two mounting holes on the IF mount.

Here's a fairly recent pic of my bench (obviously prior to the second XL650 addition)

I use T-nuts.

T-track has inherent flexibility which is undesirable to me for such a heavily loaded application.  If you use T-track, add some attachment points directly beneath the press location.  You want the loads to go from the press into the track and straight down into the bench via the fasteners.  Any offset between the press and fasteners adds additional bending moment and flexibility into the load path.  It also becomes a place where deformation and fatigue cracking can occur.

FPNI

I have a 550, Lee single stage, and plans to add a 650. I used t track on my bench but added bracing under the track and secured them with 3" long trim screws. I don't get any flex when sizing 577/450 martini cases on a Lee single stage.

Not sure if I would do it again. I like the idea if modularity, but honestly my presses stay in the same spot. I've slid them around maybe 5 times since I finishedy the bench. That might change when I add a 650 but as of now , I don't know if it was worth the effort.

On the T nuts, how do you have them mounted to the underside of the bench?  Are you using the ones with the the metal stab holders or the ones that screw on?  Amazon screw on T Nuts

Not a choice with the stab T nuts, but if you're using the screw ons, do you use a bigger hole and mount the post into the bench deck or use a smaller hole and let the post go down below the bench?

Edit: I know the post is supposed to be in the deck, but is there any reason that you couldn't mount them backwards so you could just screw them on the bottom of your bench without enlarging the holes that are already there?  Seems like the screws would hold enough to tighten the bolt and that pressure would be on the t nut, not the screws.

My 650 and Forster presses are mounted on 3/4" plywood bases with countersunk T-nuts on the underside.  The bases are clamped to my workbench  No problems at all.

Check out 80/20 Inc. I have the 1-1/2" series tracks on my bench with five different 1050s. Great flexible system that is very strong. Loosen the bolts/nuts a couple turns, slide the machines into place, tighten up the fasteners and go. There is no flex, twist or give; just a rock solid connection. I do use a stout bench. Welded steel die rack from a tool and die maker's shop that I can almost move by myself. Loaded up with the machines and about a half ton of bullets and parts, it will not budge. Well worth the $50 Craigslist buy.

I say t track. I added screws and used some good 3/4in wood screws every 4 in. Doesn't budge.

I greatly prefer T nuts vs any track system for a bench. Too much crap ends up in the tracks for my liking. With T nuts I mount everything id like to use to pieces of a "standard size" 1'x1' piece of 3/4" ply.  I use star knobs to hold them down to the bench so it's tool less to install and remove.

I had stored the tools on my wall when not in use. Was using metal shelving brackets but I think I have a better solution when I move into my next place.

I went with T track.  It seemed more versatile and I understood the concept better I guess.

Stuff does drop into the track though.

It gives infinite mounting positions, in case you want to move things around.

Don't think you can go wrong either way.

I do like the idea of the Tnuts too. Probably not as much work to install

My T tracks are very secure and the Lnl press is ridiculously rock solid.

Your tracks are installed to take advantage of the press base's leverage.  Your front track is just in front of the press mounting bolts, and your press mounting plate is large enough (and I assume in complete contact with the bench) to essentially negate any torque from operating the press.  Your plate is the key to how "rediculously rock solid" your mounting system is.

OP, if you go with tracks, mount your Ultramounts on big honking chunks of thick, furniture grade plywood.  This will not only give you the benefits of the plywood mounting plates, but will also let YOU decide how far apart your tracks should be, rather than having to comply with the spacing on the Ultramount holes.

Ultramounts are solid and extremely reliable, but having bench tracks locked in at Ultramount hole spacing may get in the way of other things on your bench, and the plywood plate will further spread out the loads the press applies through the mount.  I know for a fact that my LockNLoad press pulls up on the back bolts of my Ultramount, because I didn't get those as tight as I thought the last time I reinstalled the mount (Tee nuts here) after working on the bench top.  That was immediately obvious, and I stopped and retightened everything to correct it, but I learned just how much strain the rear mounting bolts handle.

Since OP is talking about switching between presses set up for small and large primers, there's another option: Inline Fabrication's Quick Change Press Mounting system.  The base is identical to the Ultramount, but the presses are attached to a quick change plate that locks onto the mount.  These are SOLID, repeatable, and just as rugged as the mount itself.  This would let you move the press from one location to another fairly easily - though you'd be physically lifting and moving the press, rather than sliding it sideways.

I did T-track.

I have one of the Sam's Club benches with the maple top.  I added an extra screw between each factory screw mount and have no issues with the T-Track moving.

If I had to do it again, I'd still go with T-track, but not put it across my whole reloading bench.

You can get the T-nuts for much cheaper than the link, heck they are about .25 each from my local hardware store.

That said, I just used regular hex nuts, and embedded them into the bottom of the backer board with a cordless impact and some washers.  
At first I countersunk the holes, but then I realized that my cordless impact was plenty stout enough to just pull them in...

Why don't you just install the QD base plate with T nuts? That would give you the most flexibility.

That way you can take out the base plate t nuts to move the whole mount, or you can just replace the top plate when/if you want to swap presses?

I take my QD mount off my bench all the time.

Ordered this today for my RL550b.  Getting a Co-Ax when available so this makes a lot of sense.

The other member must have missed it - I'm already using the IF Quick Change system.

I'm looking to swap out presses/tools that are already mounted on the QC system.