Just thinking out loud here - but there is some concern here about excessive cranking on a newly rebuilt engine? In this instance troubleshooting fuel pumps.

Maybe a strategy would be to rig up an outboard fuel tank (boat tank) or some type of gravity feed to the carburetor and get the 20 minute break-in accomplished, camshaft lobes and lifters "run in" and working together. Then work on the fuel pump issue.

Here's one designed to do exactly what Tedster describes.

https://www.amazon.com/Motion-Pro-08-0032-Auxiliary-Fuel/dp/B000WJ0RCA/ref=psdc_404763011_t4_B004NV42PQ





Lawrenceville, GA

I will go with "Just a bad fuel pump."  When I bought my '55  Thunderbird, it had an electric fuel pump in line to prime the carburetor before trying to start the engine.  I removed the electric pump, and the carb would not prime with the engine driven pump.  I replaced the fuel pump, and now it primes very quickly just cranking over.  The fuel pump diaphram rubber was softened by ethanol in the fuel to the point it would not create a vacuum.  Joe-JDC  

JDC