I share your love of BMWs and Mustangs so the idea of making a mustang-bodied M5 sounds great as long as you have the skills and resources. Which Fastback do you have in mind?
That v10 is a big engine-long and wide. At 500 hp and 5000 lb gross weight, the suspension and running gear will not be exactly petite. To get the running gear to fit, you might easily find yourself reworking the bottom of the car from front to rear. You might want to check BimmerForums.com to find out what is required to do a conversion.
B]2005 V6[/B] .Manual, Eibach Sport Springs, Magnaflow Duals, CAI, Tune 17/9.5" Bullit Wheels.
2003 V6 . Manual, Stalker F/R body kit, 600w stereo amp.
1999 V6.. Manual, CAI, High Performance Cats, High Flow single exhaust, Tune.
1972 Mach1. Built 351W, C4 w/shift kit, Long headers, Flowmaster 40s w/custom duals. Eibach Sport lowering springs. Kyb shocks, New paint and stripes, Custom seats, Fresh red clearcoat paint and mach1 stripes.