So I have an 84 4x4 with a pretty worn engine. A friend wants to give me his 96 Vortec 5.7 and 4L60. Anything I want from the truck in fact. It's a 2wd with over 100k. Runs great and uses no oil. Has been well taken care of.

How hard would it be to retain efi? Would it be possible to convert the 4L60 to my NP208 transfer case? Not sure I wanna give up my TH400 but lower 1st and OD would be nice. How hard would it be to convert the cooling system?

I'd like to pick up a carb type intake just in case but run efi as long as practical. Probably just put in new gaskets, seals, freeze plugs and timing chain. Maybe a mild cam.  It's just an occasional driver in bad weather, hauling stuff or shooting in hard to reach pits.  EFI would allow remote start again.  Sure do miss that.

Is it worth it?

That might be more trouble than it is worth.
Not exactly sure that the trans will bolt right up to your transfer case but I think it will.As far as I know GM uses the same length output shaft for both 2 and 4 wheel drive.
As far as what you will need from the other truck................everything if you want to keep it fuel injected.Engine,Trans,Radiator,oil cooler lines,trans cooler lines,ECM,all computer harnesses,all emissions components,fuel tank,A/C components, and you will have to rig up a check engine light and some other things.
All components need to be kept together so driveability characteristics will remain the same.

If you want to just swap engines with your TH400 still in you will have to replace the intake with a carbureted version and you may have to swap flexplates.Also use your old distributor.The 4L60 will not work without the ECM and harnesses as it is electronically controlled.A 700 R4 tranny would work if you wanted a steeper 1st gear and OD.

Hope this helps.

Its going to cost a lot more and be more difficult than you think.  Bolting it in is one thing, making it work properly is another.  

Having the PCM reprogrammed to elminate a lot of things would make the job a lot easier.

It's do-able. Many kit cars use the vette engine that is similar from the same years. Since it still has a distributor, you can install an older distributor (I believe) and run a carb with an aftermarket manifold. It has been done and, with a little fighting, should work well in your application.

However, there is nothing great about that engine and I actually prefer the older ones for various valid reasons. Including thicker castings and aftermarket parts availability and far fewer problems with antifreeze in the oil.

Now I'm looking at carbing the L31.  Anybody know if my exhaust manifolds or some headers will bolt up?

Super easy swap,can be done in a day.

4br. intake for Vortec heads $139.95
And your headers should fit fine,use these gaskets

You can make he 4l60e work,it will just become a 700r4 when you unplug it.
The shift points will likely be high\er than you want but it will work.
The down side is that clutches and guts aren't really up to the task.
Your right about having that nice big 1st and OD but you'd do better to use the 2wd tranny as a core and get yourself a good 4x4 unit.

If you really want to get some nice MPG and be damn near bullet proof get a 5 speed.
You'll already have have the truck apart and you can get after market clutch peddle kits cheap.

Thanks!

Here is the quick of it I can post more if needed.  I would run the EFI wOBDII....  If you can get the old ECM, harness, and power center it is pretty simple.  Send out or DIY the ECM to tune out things like EGR and multi O2s.  Trans would need to be torn down and swap the output shaft along the the housing.  The 208 should bolt up but I would double check transadapt could tell you more and or sell the adapter to you.

You cannot run the 4L60E without a ECM or aftermarket controller (they sell them as a stand alone controller for the "4LxxE" tranny if you are not running a PCM) as the E in 4l60e stands for electronic.  There is also no swap for the valve body to run a TV cable.   If you can't DIY the trans you'd be better off finding a 700r4 or a 4l60 and ditch the "E" trans.

The other side of carbing the setup and is also simple everything is the same aside from the head angle to the intake and there is no provision in that block for a mechanical fuel pump.

IMO either way is just going to be labor and is about the same my only thing is that you could end up with EFI and if you are not building a drag car then EFI is the way to go.