I am not sure how to explain this, but I am still using a stock replacement Autozone small block fuel pump that I've had for at least ten years and a couple of different engines. It probably shouldn't work, but it does. It has a 5/16 inlet and a 5/16 to 3/8 adapter on the outlet side. Fuel line is 3/8 except for a short section of 5/16 neoprene hose connecting the hard line to the fuel pump. I am also running a return line back to the tank. The return line is 1/4 in. with a .045 retricter. Both of the engines I've used it on make pretty good power, but somehow this cheap [ around $17.00 ] pump is able to supply the necessary fuel for a 3310 Holley up to at least 6500 rpm.