2wd to 4wd Help please

[al's71gmc]

My son and I are putting together a truck for him. He has a 72 1/2 lwb 4x4 that needs extensive body work. I've found a 2wd 70 lwb that has a rust free dent free body. I'm thinking of buying the truck and swapping the body onto the 4wd frame. What problems am I going to face. I'm thinking it will be identical exept for the running gear. Am I wrong? Between the 2 trucks we'll have motor, tranny and transfer case that we want. I know I have to change the tranny hump from lo to hi, but is the firewall etc the same or are there mods that have to be done. Any help or advice will be app.
Al