I just picked up a 4 speed saginaw at a local junkyard here. It does not look the same as the stock 3 speeds. The side cover is different and a couple other identifying marks. I looked up Saginaw transmissions on a search engine and pulled down some ID info on them along with some pictures of the differences between saginaws and muncies during that era. They are easily confused. I got mine out of a Vega. The number of rings on the input shaft shows the 1st gear ratio. Patrick's catalog states that the 3.11 low is the preferable one and it is identified by 2 grooves on the shaft. That is what I found in the Vega (Lucky I guess). I hear they can also be found in some Novas and later model Corvairs.
Shop around with the junkyard and wheel and deal to get a good price. I had heard prices up to $225 for one (with core) from a junkyard and ended up finding one for much less. Just need to be persistent.

Good Luck.

Steve
55 1st Series 3/4 ton