I have a 55 2nd series that had the 3 speed in it. I went the OD route like you are discussing. My understanding is there is a saginaw and a muncie 3 speed transmission. I had a saginaw. You have to check the side cover to check which one you have.
When I bought the transmission with the OD tail on them. It had the saginaws front. I did what Fred (rfs56trk)suggested and switched out the side covers so my linkage matched. Again, both transmissions were saginaw transmission boxes. I believe the rear section that makes it OD is muncie.
Somebody more knowledgeable will chime in.
Just an idea.
