actually the frames are going to be identical, the diesel will most likely have bigger crossmembers, different engine crossmember but the frame rails are going to be the same, the main difference between 3/4 ton and 1/2 ton trucks is the suspension and axles, 5-lug vs 8-lug, dana 60 or 70 rear vs 9 inch, wider leaf springs, heavier coils ect. ect.
is the f150 2wd or 4wd?
is it going to be lifted?
you are gonna most likely want to swap some better axles under there but without knowing what you are gonna use it for or how tall it is going to be its kinda hard to see what you are doing...