Alan,
i presume you are referring to an e28 m5. i have no idea what it would take to fit it into the coupe ... or what the geometry differences would be. my guess is that it probably wouldn't react the same in the coupe as it does in the sedan.
i saw a 2002 here in Atlanta that has an s14 engine from an e30 m3 along with the majority of the suspension grafted in. a mechanic friend of mine tried to sort it out, and other than putting the e30 brakes + hubs on it ... he thought it caused more problems than it solved.