[Yukon Jack]

The 4 speed should be the SM465 and all you need to swap it in is the correct adapter. I would imagine Advance Adapters could help you or you can get one from a junk yard or possibly the parts board. In addition to the items you listed that you will need, you will also need the pilot bushing.

Someone else were verify as I don't know automatic info very well, but I think you can use your existing transfer case crossmember, may have to move it back or forward though.

Again I don't know automatics but I assume you need a different flywheel and of course you will need a clutch, throwout bearing, clutch fork and related clutch linkage.

As to whether it is a good idea, depends on what you are trying to accomplish. The SM465 is ultra strong, but the same final 1:1 gear ratio as the TH350 and TH400 for that matter.

You won't use the 1st gear in every day driving, but that low gearing is great when needed.

I'm pretty sure the driveshaft issue is answered in another post, but that might be difficult to find - someone will let you know on that issue too I'm sure.