I didn't have to do any real modifications to put the 4-speed Saginaw (Camaro trans) in my 1956 3100. My 350 has a V8 truck bell housing so I'm hanging the Saginaw off the back just like the SM-420's were done. I installed a T&F Shifter that puts the shifter in the original 4-speed location with only a little trimming to increase the floor hole opening.
T & F Shifter
31799 Chestnut Lane
Lebanon, OR 97355
541-451-1929 Mark
I don't think he has a web site...
Mike B
