About the only information I found in a manual was that the pressure should be 3-4 pounds. Actually, both the old and the rebuilt pumps had around 4#. I seem to remember that some of these old carburetor float needle valves can not hold back 5#. The flow test was at cranking speed, less than idle, so you are probably correct in that 6 oz is low. Suppose I could rig up a temp fuel supply to the carb and measure again. Maybe if the problem comes back.
I really like your SWAG. You are correct in thinking that 60 MPH is pushing it. The engine is screaming around 50 MPH. My gas mileage has been between 14 and 15 MPG. 12.8 oz/minute does seem a little low, especially if that one pint in 30 seconds is correct, yet I have put the engine back together and it seems to run just fine. Time will tell. AND, after closely inspecting the inlet check valve, it was definitely leaking. Thanks for the help, MIke