Filling the top?

I am working on my first real street rod. A 35 Buick. I want to put a solid top on the car rather than replace the canvas top. Everyone says to use a van top. Is one van top better than another? Do I cutout the top larger than the hole an flush weld it or try to fit it in exactly to the existing hole and weld up the seem? If anyone has done this please pass on some advice.

any non ribbed van top will do, I'd fit it exzactly into the hole but ither way could do.