Don't try to cut it to the exact size at the boneyard, cut it a little larger so you have some trimming room when you start fitting it in place. I think I would cut it big enough so that once you find a roof with the right curvature you can lay it on top of your car and go underneath and trace around the edge. Or if that isn't feasible you can at least have enough material so you don't have any gaps to fill from cutting it too small the first time.

Car is looking good BTW.

Don