Do you have a old stock oil pump drive unit? If so have the teeth removed from it and install it into your block with that pump and use a long handle screw driver to rotate the pump counter clockwise to check for binding
Is this a internal pick up oil system oil system or external pickup system? If you are you using the stock pump with the stock cover it shouldn't matter how it spins in the block compared to out of the motor, but if you don't check it both ways you won't know if it does have a problem installed in the block or not
I built a bracket race motor several years ago with a new cast iron 4.500 bore World block and a Milidon dual line external oil system that worked fine on the engine stand when priming and checking, worked fine for 30 minutes on the engine dyno and then twisted the new Milidon shaft off in the pump rotor Luckily it was running under 2500 RPM warming up after lunch so no damage was done to it That dang block had a .003 machining ridge on the oil pump mounting boss that ended binding that cast aluminum Milidon oil pump body enough to bind and twist off that shaft when warmed up the second time
Good luck on your deal, let us know what you had to do to fix it