The FD 500/4.5 L was one of the latest designs of the big teles for the FD system, and was in fact the same lens as the first version for EF mount. Given the cost of these lenses, it would be worth purchasing an FD body just for use with this lens.
Canon did make an FD-EOS converter, which acts as a 1.2X extender, for lenses longer than 200mm, but these are expensive and hard to find, though I do have one, and it's not for sale.
I use the FD 600/4.5, which some people think isn't as good as the L lenses in the FD line, because they have never actually used it and are only looking for the red stripe. The only problem I've had with this lens is some chromatic aberration that can be visible in high contrast scenes with a 2X extender. This isn't an issue that I can see with the 1.4X extender. Here are some typical shots with the FD 600/4.5--
The FD 500/4.5L is probably a sharper lens, being of later design, but for birds I'll take the extra 100mm.
With any of these lenses, the tripod and head are more important in terms of sharpness than the lens, so I think it makes sense to save some money on the lens and put it into a tripod and a head like a Wimberley or Arca-Swiss B2. The manual focus versions are also considerably lighter in weight than the autofocus versions, even more so if they have IS, and a good tripod is better than IS, presuming you don't have to handhold it on a boat.