When you rebuild the pump, since the block oil system isn't dry,
Just gob in a handful of petroleum Jelly or break in lube.
Since the engine bearings are NOT dry, you don't have to prime the engine...
The 'Goop' in the pump will make for suction VERY quickly and protect the pump for the one or two seconds it will run before oil gets there.
What I would do is take the filter head off the pump, and check the bypass valve.
That pressure bypass valve needs to open to let excessive pressure off, and it sounds like it's not working.
When you take the filter head off, your oil pump impellers can fall directly out of the pump hosing, so be prepared to catch them.
Oil pump impellers are NOT timed, you can put them in any which way,
Just make sure the one with the long shaft engages the distributor shaft before you try and install the floor plate/filter head again!
I've never heard of a production pump producing 120+ pounds of pressure,
and if it's true, your distributor drive gear on the camshaft won't live long with that kind of resistance against it.
Again, I urge you to get a hand held gauge, thread into the sending unit port, and get an actual reading...
120 PSI should have split your filter wide open,
And that is what makes me wonder about the voracity of your "mechanic's" claim he put a mechanical gauge on it and it read 120 PSI...
Let's start at the filter,
Just above the filter there should be the 'Filter Head'.
That filter head has a bypass valve in it.
Normally, when the filter gets clogged, that bypass valve is supposed to bypass the filter to get oil into the engine.
When enough back pressure builds up, (Resistance from a clogged filter),
A preset spring pressure is overcome, and the oil is allowed to circulate in the engine...
Bypassing the filter.
It's a 'Fail Safe' system if the filter decomposes or gets clogged, oil can bypass the filter and reach the engine.
(Dirty oil is better than NO OIL!, that's why the call it a 'Fail Safe', even if the filter fails, the engine is still saved)
I don't know how that valve would fail to give you 100+ PSI oil pressure, but it's a place to start!
The only way I can think of for your oil pressure to be reaching 100+ psi is for a main feed to the engine to be BLOCKED,
And that would mean the engine isn't getting ANY oil volume.
The AMC V-8 is basically one HUGE leak from a thousand tiny holes...
The block is like those garden hoses you water with, they are punched full of little holes all the way down their length...
Front cam bearing should be changed ANYTIME you have the front cover off,
Since the timing chain, fuel pump eccentric and distributor all load the front bearing, it wears out VERY quickly...
Then you have camshaft bearings that run continuously,
Then there are the lifter bores that are constant sources of volume/pressure loss.
When you get into the bottom of the engine, there are rods and mains that drool like mad dogs in heat!
We all know about the loss of pressure to the back of the AMC V-8 blocks,
And lack of volume from all the 'Leaks' is the reason why!
AMC V-8 blocks aren't like SBC blocks, you can't connect directly into the gallery plugs at the front end of the block and run a pre-oiler to the internal oil pump...
Because AMC's have a front cover that holds the distributor/pre-oiler that has to be off the engine to access the front gallery plugs,
AND, AMC's don't have an internal oil pump!
(and people wonder why I install a second gauge in the BACK of the block when they are down for machining! So you can see what's happening at the END of all those 'Leaks' in the engine!)
I don't know how the bypass valve would fail to cause 100+ PSI...
But it's a place to start.
Since the filter didn't split at the 'Gauged' 120 PSI, and should have,
I would get a mechanical gauge, and plug it into the oil pressure gauge line fitting and check for myself...
I've never seen ANY filter that would take 120 PSI, and I've blown quite a few up with these racing engine oil pumps...
If you didn't see the guy do it with your own eyes, then do the test for yourself...
When you take the filter head/floor plate off the pump, the Impellers WILL fall out in your hand, so be ready for that!
Also, you need to 'Dry Fit' the impellers in the housing using 'Plasti-Gauge'.
Good rebuild kits will come with two or more thickness gaskets, start with the thick one and work down until you find the gasket that gives you the best fit,
And use Plasti-Gauge under BOTH impellers, they are often different thicknesses!
Good rebuild kits come with bypass valve spring & plunger/piston.
If you don't get one, you will have to track one down that DOES have the bypass rebuild kit.
When you have figured out which gasket to use (DO NOT stack gaskets!),
Don't forget to pack the impellers/housing with Vasoline or assembly lube so the impellers don't DRY START.
DO NOT USE GREASE!,
Vasoline or assembly lube will dissolve in oil, where grease chunks can clog small oil passages and build up gunk in the engine.
This is from the link someone posted, and it's got the bypass spring and plunger in the middle...
BUT I ONLY SEE ONE GASKET...?