I have a couple AC Delco SBC fuel pumps. One is short and stubby like the higer pressure and flow aftermarket jobs. The other is bigger (that can thing on the bottom). Are the short stubby ones generally higher pressure and flow or is that just a coincident. I cant find any part #'s on these things to identify them. I would like to get one of those 100ish gpm @6psi models. I have a couple laying around and would rather use them that buy a new one.

 

Without a part number, you really aren't going to know with any certainty. However, a larger diaphragm generally will provide greater flow, but not necessarily greater pressure. Look for the one with the larger diameter in the crimped area of the housing.