If I already have an extra 5.56 lower, what do I need to to other than buy a 6.8 SPC upper.
I guess I would have to have 6.8 cal on the side but will the mag well transfer over without
a problem. Will I just be able to buy 25rds 6.8 mags and go to town.

Thanks in advanced for the help,
D

I just built one using a LAR "multi-caliber" marked lower.  But you dont' have to take the 5.56 marking off your lower, as long as you own the gun and make sure you know it takes 6.8 cartridges.  All 6.8 barrels I have seen are marked "6.8spc" in a location that can be seen when the upper is in place on the rifle.  If you were going to have the firearm in a location where someone else might pick it up and try to fire some 5.56 thru it,  I would put a small piece of tape over the 5.56 and mark it 6.8.
As for the mag well, its the same.  All they did was make the internal portion of the mag wider by removing some grooves from the side of a 5.56 mag and changed the follower.  The 6.8spc 25 round mag is the same width and height externally as the 5.56 thirty round mag.
hope that helped....
be safe,
thomas

BTW, you will love the 6.8 too.  I don't know what type lower youre putting on, but I built this one using an FN flat-top upper, an ER Shaw barrel, free floated it with a Yankee Hill lightweight float tube, and installed a Rock River two stage trigger.  After breaking it in and scoping it, it will shoot Sub-MOA, at times its all one ragged hole for five shots with Remington Match ammo...about 1/2 moa.  Will make a perfect treestand gun for deer.
have fun,
thomas

Is there anyone producing 6.8 SPC-marked lowers that have good availability?
(that can be purchased as separate lowers, not as an entire gun)

I've had a hard time finding any.

I haven't looked at them all but I do know that Bushy offers a 6.8 lower.  Though there are some out there that would turn their nose up at a Bushy.