Small Block Mech Fuel Pumps?

If the M6903 is in stock, I'd give that a shot.
Both that Carter and that Holley have both ports as 1/4" NPT.
If you want a little less resistance to flow, chamfer the fitting or adapter before installing it.
Not sure what was on your old pump. May need a 5/16 hose to 1/4" NPT on the pump inlet, and a 1/4" NPT to 5/16" Inverted Flare on the outlet. Unless you increase the size of one or both to 3/8 tube/hose.