I have both a 70 Mustang and a 72 Comet. I can tell you the exhaust from a 70 Mustang will not bolt directly up.
I used a kit for an early mustang. Had to dent the corners of the rear footwells in one or 2 spots and shorten the bent pipe. Nothing major. http://www.summitracing.com/parts/wlk-89025/overview/