I have done what you want to do several times but you need to start with the frame for a 73 or later 3/4 ton truck.1/2 ton frame will worktoo if you want to use 5-lug bolt pattern wheels instead of 8-lug wheels. Then graff the two frames together just forward of the front cab mount.I usually install a scab plate on the inside of the frame channel ,then weld a boxing plate about 16" or longer, running with the length of frame so that the weld joint is in the center of the boxing plate.
