Gary_P is right again, both early & late heads will work on the late block. It's only the late head/ early block combo that's a problem.

The oil pump lays on its side, so it is difficult to prime with oil & have it stay there, so I use a light grease & push some of it down the inlet side while rotating the shaft by hand to ensure that the interior is coated with it.
The grease is used as a sealant to help the pump to move the air from it & pull the oil up into it. It is also very important that the pickup flange is flat, & that the seal between it & the block is a good one. If not, then at cranking speed, it will suck air into the pump.

I have a very small oil pressure guage that I bought at a Harley shop & I fit it directly to the pump & watch it while I crank the engine over [without plugs] with a remote starter button, & I never start an engine untill I have at least some oil pressure first.

Oh yeah, you did fill the filter with oil before fitting it, didn't you? Even a running engine will need a few seconds to fill it if it's installed dry.

My only problem with A series engines is the inability to drive the oil pump with an electric drill to prime the pump & build pressure before start-up like you can with L series & my Chryslers.
Don't wory about your oil pressure guage line. It does not need to have oil in it for it to work.