Cause for low oil pressure other than crank plugs?

[Janice & Steve Ashby <sjash@domain.elided>]

Hi,

Just pulled the engine from my new 74 GTV to replace the crank plugs and I
find they all look pretty good.  Now what?  Being a know-it-all, I didn't
even put a known Oil Pressure gauge on it before I took it out.  It had the
standard symptoms, decent pressure at startup, dropping to very low after
warming up.  Since the car shows 89K miles, I was sure it was the plugs.
What I found was all of them except one look fine, the one had obvious been
replaced.  It was just completely peened over (about a dime sized glob of
aluminum left on the top) but seemed pretty solidly in place.  

Is this behaviour indicative of a bad oil pump also?  Worn mains? How do I
check the oil pump (I don't have a .0024 feeler gauge)?  Should I throw the
engine back in and check the pressure with a good oil pressure gauge?
Advice requested.

Steve Ashby
74 GTV (as above)
75 Spider (sat in the rain all day to clear garage space :(  )

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