I can't give you casting numbers but I can tell you the 3.11 first gear Saginaw trans was used in 4 and 6 cylinder cars from GM, mostly like the Chevy Vega, Monza, and similar bodied Pontiacs of the 70's and early 80's.
The 8 cylinder version of the Saginaw had @ 2.54 first gear and is more commonly found in G bodies like the El Caminos until 1981 and in the F body cars, Camaros & Firebirds.
If you are lucky enough to find one still in the vehicle this may make it easier to know which trans it is.
