There should be a ton of Mustang owners out there that would be willing to trade a C-4 Auto setup for your existing 4-Speed and all of the related components. I would rebuild the C-4 in any case. They are relatively cheap to do (about $450 to $650). I think the C-4 is a great tranny and ran one 180,000 miles with no problems, then rebuilt it just because it was old. Let someone else worry about clutch discs, throwout bearing etc. I think autos are great and especially if it is a real daily driver. Good luck.
65 Conv., 65 & 66 Coupes and the remains of
a 66' Coupe with 6 cyl. engine and V-8 3 spd.