Good info. here. My '46 ton-and-a-half has a built-up bed, also.
If you build the bed off-truck, make sure you have some way to lift it up so you can get the truck under the bed. One way would be to build it up on cribbing, with some judicious planning, back the truck under the finished bed and bed frame, and lower the whole shebang down to the truck frame.
Don't forget to check your state requirements for things like clearance lights and mud flaps.
And don't construct a bed too long for the truck frame, you want to maintain weight balance and gross limits. Anything over 12 feet could become problematic.
Send pictures as you progress. G.L. will gladly tell you exactly what you should be doing at each step!
