It would probably be much easier to just replace/rebuild the current trans in the truck.
The way the newer auto transmissions (4L60e and 4L80e) is completely different from the th350 and may require changing/reprogramming the computer. You would also need to look at the driveshaft length, crossmember, and how the shift linkage is hooked up. None of these are insurmountable, but they may make it more trouble than its worth.