I had a stock non-syncho 3 speed behind my 1966 I6 200 engine years ago and had nothing but problems. I converted it to a 4 speed toploader and have been thrilled ever since. Here is a list of parts you will need:
157 tooth aluminum bellhousing and release fork from 1967 I6 200
157 tooth flywheel
matching clutch, pressure plate, throw out bearing and input shaft pilot bearing (presses into flywheel center)
toploader 4 speed and shifter lever
A 1965 4 speed toploader bolted up without a problem.
I just removed all of the above 6 cylinder parts for a V8 conversion and don't need them anymore (minus the 4 speed itself). PM me if you are interested. I will sell everything cheap. I am not looking to get rich but just want them out of my garage and in another mustang.
1966 Coupe - V8 Conversion