74 318 fuel pump interchange with 73 340?

Can't seem to build any fuel pressure on rebuilt 340 currently using 318 fuel pump. I didn't think there was a difference other than overall pressure, right now not even a squirt but was running on 318?...

got gas in the tank? What was wrong with the other pump? Put a rag in the filler and stick a air nozzle through the rag and blow compressed air into the tank to pressurize the tank and get the fuel flow started.
or pour a little gas into the carb and start the engine. Sometimes this speed is all that it takes to prime the pump.

If that don't work, how long did it sit since it was on the 318? could have been ready to go then. Dried out and cracked