I recently bought some seat mount parts from Pat Wilson's T-Bird shop in New Jersey, as they sell used parts along with new parts for these cars. You should find them with google easily. The guy at Wilson's will tell you EXACTLY what you can or can't convert or and what parts you'll need.
With the manual seats there are inner and outer seat rails, a couple of springs on each side and (if you want them) skirts that will cover the rail assemblies. I feel fortunate that my '65 has no power seats, windows, door locks or remote trunk release, so I understand your desire to change over.