Sean-
This discussion comes up often. There seem to be several important factors. One, the plate-to-plate width needs to be pretty close to your exisiting rear end. Second, most indicate that gear ratio is key for a smooth low rev ride after the swap. Third, check out the rear end U-joint measurements as you try to match the output from the new motor-tranny. You may need to search for an appropriate driveshaft as well. Finally, consider all the "mating" you have to do with systems (brakes, shock mounts, etc.).
See this page for more info:
Chevy Trucks Rear Swap Info
