power steering pumps

Does anyone know what the difference is between the TRW/Thompson P/S pump and a federal P/S pump. I understand about the gpm and what the back of the each housing looks like but what is the difference? The mounting holes look the same, the reason i ask is because there seems to be no one making or has brackets for a 69 and old model and all of state will not fit a federal pumps so what gives? I'm talking with another member about buying his set up but from what i have read the federal pump seems to be a better set up over the saginaw. Right now i do not have power steering on the car but do have most of the parts.

Thanks