I imagine you have a very large pile of "bits" then. The M5 was based on the 535 - but LOTS of bits are different - interior, steering box, SLS (then again some 535s had SLS didn't they?), roll bars, springs, shocks, brakes, exhaust, fuel system, body trim, lots different under the bonnet (hood) - eg oil cooler, expansion tank, air filter setup. I also found plenty of wiring differences between the '89 535 and 90 M5 - eg some lights would not work when I swapped LKMs - no CCM in the 535. It can and has been done. I believe one in NZ was done on a 525.