I rebuilt the front center window on my '73 (which I expect is the same), and used 1/8" polycarbonate. I don't have the exact height and width, but if you remove the old piece, you can get a good measurement off of that, assuming it stays in one pice. If not, then you can at least measure inside-to-inside of the frame. If you have a table saw handy, you can just buy a 48" x 24" sheet (which should be over-sized), and then cut it down to fit. Lexan cuts real easily with a finish blade.