A drive shaft from an e36 328i will fit the transmission output shaft\differential input flange and the length will be correct. The 328i was equipped with the ZF transmission same as the M3.
I used a 93 e36 325is drive shaft and it was the correct length for my car. It only required a slight adjustment in the length at the spline. This was done by loosening the compression nut on the spline and adjusting the length on installation. I'm using a Getrag transmission so it was a direct fit for me, no problems.
Below is a link to an e30 forum thread on this subject:
http://www.e3024v.com/board/index.ph...age;topic=14.0