1974 D300 360 fresh rebuild will not start

Cam really should be degreed in.
I have seen it 4 separate times where the dot on the upper gear was on the wrong tooth(cam was retarded a tooth as a result in each case).
If you still have the old gear you could compare the two.

The 60psi cranking pressure is still the problem that needs to be solved.

As was mentioned, verify with another gauge.
Tested with all plugs removed, carb at wot, battery charger hooked up, test 4 pumps, and then again at 8 pumps.

If you are positive the gauge readings are correct, and are still only 60psi, then the cam timing really needs to be verified(degreed).

If the cam timing ends up being correct....... then I’d say it might need to come back apart.

That would be a situation I have never seen.........a re-ringed short block that has such bad ring seal it will only crank 60psi.