Wanted to answer my original question for any future searchers. Got the Sniper back from Holley, ( They wanted to send me a re-furb, but They agreed to fix my zero mileage one), and it started up first try. Installed a pressure gauge between the stock, mechanical fuel pump and the Powersurge. The mechanical fuel pump's return port is closed and the Powersurge overfill is routed back to the tank via the stock return line. Ran the engine at various engine speeds and the maximum pressure indicated was 3.5 psi. So...this application will not overpressure the mechanical pump diaphragm. I did not close off the return line to see if the pressure would build, as I decided that wasn't a likely scenario.