If modifying witjhout additional glass (expensive and not so good in terms of optical results), you need to recover 2mm somehow to reach infinity focus, and that could be done by removing FD mechanic...
See what I did:
http://www.iceinspace.com.au/forum/s...anon+50mm+lens
http://www.iceinspace.com.au/forum/s...s+modification
http://www.iceinspace.com.au/forum/s...s+modification
http://www.iceinspace.com.au/forum/s...ht=Canon+100mm
Also, older manual m42 lenses (Zenith, Pentax etc) are the easiest - you just need $5 adapter from ebay and you are done.