If the rear end in the truck is original, it is either 4.57 or about 5.10. I think the 49 had the same driveshaft as my 52, front half closed, rear half is open.
You can put the 1/2 ton rear end in, sounds like it's set up for the open driveline. You will probably have to get a crossover u-joint, I think they used different sizes between the 1/2 & 3/4 ton. Most parts stores should have one. Just take in both joints and tell them what you need. You might have to do some work with the spring perches, don't know for sure.
If you go to the tech tips section, you can find instructions on how to determine your gear ratio.
