Hy dads51, that sounds like the three speed used in the 55B or 2nd series through at least 59, I'd have to measure one to be sure, the 55 - 57 Chev cars used the same transmission, the later car transmissions were shorter. I tried to find the designation for it, but I can't, I think it might be called an SM318 but I'm not sure, hope that helps.
