1978 250 Inline 6 cylinder oil pump issue Advise

[Kimble005]

So, I rebuilt my dads 1978 inline 250. Question is....why would the oil pump take so much force to spin? When I took it out of the box before assembly, it spun fine. Now its installed and when I initially primed the engine, the drill struggled with spinning it. And when I stopped spinning it, it spun backwards for a moment or 2. Now since break in is over, I figured it would loosen up, but it hasn't. It will actually almost stall the engine when 50 lbs is reached. Any thoughts or advise would be appreciated. Thanks in advance.