you know, i was looking at a 75 or so dodge 3/4 ton 2wed pickup that we were scrapping not too long ago and the box is a saginaw and identical in appearance to wehat GM trucks have and i bet it might work, as long as output splines are same and as long as input sdhaft is same length and everything, and also mounting bolt pattern four bolt holes too, im sure they are.
get the casting number off the box housing and compare to the numbers on a gm one that would be a good start,