Well, first I don't mind to question your abilities, but it requires a lot of skill in sheet metal fabrication. You would want to make a cardboard template of the roof curvature just below where you want to cut it, then see what junk yard prospects that the curvature would match up to that are flatter than the Buick. Or, just cut the roof below the big crown, and fabricate your own top. With the compound curves on a roof, this would be best done with an English wheel..... Lots of cutting, fitting, shaping, hammer welding, and frustration to get it done.......