If you have any old (eg dead) ATX boards laying about, maybe strip down the Mac and have a look at how it might fit in there. It's certainly a decent size case, so the question would really be how you get around (or remove) internal obstacles, route cables, and what rear panel mods would be needed.

If you don't have a dead/disposable ATX board, it should be easy to rustle one up from a nearby retailer/repairer/dumpster.

Now would be a good time to invest in a Dremel >