Only thing I would add to Tech's advice is before you remove the sheetmetal, engine, or anything from the Roadmaster donor is to remove the front shocks and make a set of tubes to replace them that are the exact same length as the installed height of the shocks when they are in the car. Easiest way to do this is to put a tie wrap around the shock rod and push the tie wrap down to the shock body, then when you get the shocks removed, measure the length of the shock from mounting point to mounting point with the shock compressed to where the shock body just touches the tie wrap. Build your tubes to this same distance between mounting points, put them back in the car and tighten them up. This will retain the correct angle of the lower control arm.

You will have to have the engine and trans installed and the body on the frame to set your ride height correctly. Once things are tacked into place, you can remove the body, engine and trans, and all the rest of the stuff in your way so that you can get at everything to weld it properly!