Good luck with your project, post lots of pictures and take your time.
The transfer case is often overlooked on a lot of power swaps. Good on you for thinking about it! From the factory, the transmission and transfer were both shifted at the same time, via the shift tower. When the shifter arm was placed in REVERSE this not only shifted the transmission, but simultaneously shifted the transfer in reverse which also powered the front axle for 6x6 in reverse. As soon as the shifter was placed in any forward gear again, this would return the transfer case to "forward" operation which also disengaged the front axle. So under normal driving conditions (smooth pavement) your only being powered by your rear wheels. This is how it works 99% of the time. The only time the front axle gets engaged is when the rear wheels begin to slip by as little as 6% which causes the forward sprag unit to transfer power to the front wheels as well. As soon as the rear wheels slowed down enough, the front axle was disengaged. So this, and when the transfer is in REVERSE are the ONLY times that the front axle is under power. It's a little confusing at first, but once you understand it you'll never forget it.
Now onto your question, when you change transmissions you obviously lose all of your shift linkage. Except for what is connected to the Hydramatic crossmember. If you can keep the original crossmember and all it's transfer case linkage, it will make rigging up a lever to control the reverse shifter shaft very easy.
So you have two options. If you don't want to mess with the linkage, just drop the front driveshaft and ignore the front output on the transfer. However if you want 6x6 capabilities, the linkage is mandatory. Failure to shift the transfer case into forward or reverse can cause the forward sprag to bind and eventually break.
Throw all the power at the transfer case that your heart desires. It is VERY VERY strong. The original 302 made 260 ft lbs of torque. The Hydramatic had a 4.54 reverse gear ratio with a low range reverse ratio of 17.35:1. Do the math, that is 40,000 ft lbs of input torque on the transfer case. With these ratios, it's no wonder the GMC's were notorious for snapping so many driveshafts and axle shafts like dry spaghetti.
