whats the difference betweem gm and ford saginaw box

Possibly different splines on the input and pitman shafts. Also perhaps different internal gear ratios.

I have both of them at home. I have one I just picked up from a Ford Van, 80's vintage. The other on is standard GM issue. I'll take a look this evening to see if they look the same as far a mounting points go. I could not answer the question if there is different flow characteristics between the pumps. I assume there would be but I have no Idea. I am going to use the GM pump on my car since I am using a Steeroids rack (J car).

I can tell you the the shaft on the two pumps are different. The GM rack uses a 5/8" bolt to hold it on and the Ford pump is a press on fittling. I am not sure on size but it looks like the same size as the 5.0 Mustang pump (11/16"). I'll check that this evening as well.

I don't know if this helps but at least it is a place to begin the discussion.

shoguun,
All I can say is:
1965-04 Mustang
Click on their "Catalogue" for pricing. Best option I've found. Read all their info. You can't use it with a stock clutch "Z-bar" setup, but hydraulic or cable is easy if you have a manual transmission.
HTH
Gene