The C4 that you need is the type that has the filler tube in the transmission's case. It's referred to as a "Case Fill or Step Case". The other design that can't be used on Mustangs is the "Pan Fill", with the filler tube attached to the fluid pan. The last year for the C4 trans was 1982. The stronger and improved (Fluid routing) C4s are the ones built from 1971 and up. 1970 was a transition year with a few unique for 70 parts. The valve bodies on these transmissions have a manual shift mode that lets you shift 1, 2, 3. Drive is 3rd when you manually shift. This shift pattern started being used in 1967.
The 1965-66 valve bodies had a different shift pattern that is considered less desirable for performance use because there is no manual shift mode. There is a low or 1 and a 2-3 pattern ( Big Dot) and a drive pattern with 1-2-3 (Little Dot). I'm not 100% sure, but I don't believe that the 65-66 valve bodies with the related shift pattern will work with the newer C4s that have a full manual mode.
Another issue with C4s built for Mustangs is the cable operated downshift. This was discontinued after 1968, so a later C4 will need to be retrofitted with the lever for the cable.
As long as the transmission is a case fill, all of them up to 82 should work.
Any 72- later C4 with the dipstick in the case (not the pan) that is from a 250 six cylinder car or V-8 will fit and be good for mods that make it a real performer. There is lots that you can do to make it stronger but the first step would be a TransGo 40-2 valve body kit (for 72 and later) and a BIG cooler.
If you need help let me know, I have been building performance C4s for 35+ years.
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