Air has nothing to do with the oil gauge. If you actually have pressure, the gauge will read, air in line or not. You sure you are not over-tightening the line fittings so much that the compression is squeezing the line?
I'd remove the line/ gauge, and hook them up to an air source, regulator or inline valve if possible, and "see what you have."
Most likely plug left out is the "trick" one that is INSIDE the engine down by the distributor bushing. You can see that by pulling the dist. with a flashlight. There's been a couple of members on here actually got them back in through the dist. hole. If not you could pull the intake
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