FD to EOS converters (they have relay lens to achieve focus at infinity) may add their own distortions.
Perhaps it may be better to try something like I did:
http://www.iceinspace.com.au/forum/s...ad.php?t=39293
http://www.iceinspace.com.au/forum/s...ad.php?t=51866
The modification is not necessarily irreversible, if performed correctly and carefully (but I do not know why anyone would want to go back..)
If you have M42 lens, then M42->OES adapters work fine. Russian prime lenses (for Zenit cameras) are usually good value for money.
Good luck..