Try LateModel Restoration Supply
http://www.50resto.com/ , they seem to have most of the things available for older Mustangs. Their newest catalog is very informative, just got it a couple of days ago. If they do not have something, it probably isn't being reproduced. I have not had any major probs with MPS, any probs I had were promptly resolved.
Is your car carbureted or does it have CFI? The clutch is upgradeable, from 10" to 10.5" by replacing the flywheel and clutch assembly with 86-93 components.