Has anyone changed their Auto to a 3 speed???

Why not swap in a toploader 4 speed? I did that swap several years ago and it was pretty sraight forward. The parts are different if you have a I6 or a V8 though, but either way it can be done pretty easily. The main thing is you need to get a clutch pedal, you can use your existing brake pedal support member since they are the same for manual and auto. The trans mount is the same too. The key difference in V8 vs I6 is the bellhousing. For a 3 speed I6 you will need a 67-68 4 speed bell housing. If you have a V8 then you need a V8 bell housing. Another important difference is the year of the engine 66 vs earlier. The 66 has a 6 bolt bell housing while the 64-65 has a 5 bolt bell housing. I once found out I had a 65 engine in a 66 Mustang and when I swapped transmissions I had an unpleasant surprise.

Another factor is price though; a toploader is pretty expesive and you need to get the right one. I paid arounf $700 for a used one and then had to pay over 200 for a Hurst shifter.

I do have a 3 speed transmission that I recently swapped out in another Mustang for a Toploader 4 speed. It worked great but I was rebuilding my engine and figured since it was out it would be a good time to swap out the 3 speed. It is complete with shifter and if you want it I'd sell it for $50.00 plus shipping. I have it laying around and am trying to figure out what to do with it; I'd hate to trash it when someone could use it. Just PM me if you are interested. If not that's OK too. BTW- I also have an old Mustang Monthly Article on PDF that I could send you that shows how to swap out a I6 "paper" 3 speed trans for a V8 3 speed trans. It may not be exactly what you are doing but it does show the general process that you will be doing swapping out the auto. It also has a side bar on how to swap in a 4 speed toploader.