The best way, and maybe the hardest way also is to get a doner car and cut off it's roof. Then weld in some braces in your car from quarter panel to quarter panel to help hold the car together, then weld in the new roof along factory seams. You might think, "I'll just weld in a patch panel" but you will never get the contour right, and the metal up there is thin and might not weldup well, you will probably get a lot of burn through. Also if you don't get the body work right it will look wavy at best. Don't forget these cars are low, and the roof is the first thing you see when you walk up to it.