First link will give you the differences for the 64 1/2 mustang, 2nd link is probably the largest parts supplier for classic mustang parts.
Hope that helps!!
Refer to the site below for 64 1/2 differences.
You don’t say which transmission you have, either a C6 or even a C4 automatic, the 4 speed top loader or the 3 speed. There are plenty of parts available for the excellent Ford 4 speed as it was an excellent transmission, equal to the best of the era. I have included two sites for the automatics. the last is for a top loader.
I only hope you don’t have the 170 ci 6 cylinder. This engine was not only a dog but had 4 main bearings and was prone to early failure. The 200 inch 6, in contrast, was a great engine and very smooth. If you have a V8, so much the better.