I have a 1963 pontiac grand prix with a 421, which needs a new transmission. The trans in it now is not original, but I'm trying to replace it with one that is. I believe the original automatic that would have been in the car was a slim jim. Does anyone know if the same slim jim transmission was used with the 421 as well as the 389? Thanks Rob

Hope this helps.
http://www.rodandcustommagazine.com/techarticles/82698/index9.html