Well, from another guy that has had both 2-1/2 tonners and 5-tonners, asnd many years of time in U.S. MV's of other kinds....
All the before comments regarding the 2-1/2's vs 5 t's are applicable. Especially the part about the 5t's being just plain big trucks. (Yes ten lugs at 450 Ft-lb and 251# each can make you talk to yourself.)
The 2-1/2 is a much more "civilized" truck. Big, tough, off roadable, almost indestructable, fun and realistically sized. They go right at 13,000 pounds...the 5t is 9,000 on just the front axle........
Both have compressed air system for brake boosting, the air-o-matic makes the 2-1/2 just about as easy to steer as the PS on the 5t. Again, no CDL required for the 2-1/2. There is virtually nothing that the 5t has that is not standard on the 2-1/2 (not counting PS). If you got with the M-35 variants the cabs and all options there are the same as the 5t, if you go with an older GMC such as a 211 then it is a different case, they hava auto transmissions, bigger cabs, heaver drivelines and only 302 gas engines.
Really, the 2-1/2 t is a much better candidate for modifications, and you can still sit-up under it to do most service.
Squirt Truck(er)