Jesus...tough crowd. I'll try to help then. The link below will take you to the generally acceptable tire/rim combos for a 65-66 mustang. Of course you can go wider than what's on here, but may require rolling fenders or bodywork to fit without rubbing.
I believe 17x8 with a 255/40 is the widest you can go on a stock 65-66 car. Now if you have tubs (or mini tubs) and/or massive fender flairs, and a suspension to go with it, then you can go as wide as you want.
Figured you’re asking on the widest possible wheel on a stock suspension.