Be careful w/ the 6spd. I had/have the 59 psgr. car 3sd OD that came w/ my 352 mtr. that I planned on using, but I studied the ratios and determined one or more of the gear "splits" overlapped and would not give me 6 useable ratios.
Further study showed a med. duty later (69?) F100 3spd od would give 6 nicely spaced gears. And it proved out. Been running it for years.
I have all the ratios and casting # documented if you need them.