It is not uncommon to see one of these cars with the hole in the dash all hacked up. People used to do that to install a variety of size huge tape players 'back in the day'. The optimum solution is to cut the area out and weld in a piece that has the correct holes. This 'piece' can be cut out of a basket case donor car, or, new replacement panels can be purchased from most vendors of vintage Mustang parts. If your budget doesn't really allow for that, here is a good alternative. It would not pass muster at an MCA car show, but, the piece actually looks pretty nice, and doesn't call for any cutting, welding, etc.. It would be something well worth considering.
http://www.mustangdreams.com/repair%20plate.htm