I have several FE’s, only the 390 in my old pick-up has press in plugs. I have repaired/built a number of them over the years and in my experience the press in gallery plugs are not as often a problem as the cam plug. It was common practice for rebuild shops to slather some silicone sealant or some other goop all around the cam plug because they did not trust it. (I have seen several of them installed backwards and if this is the case the cam can bump it loose) On assembly I use just a bit of red Loctite on the rim of the plug, not smearing stuff around the plug.
I recently repaired an FT 391 (same
oil system) that the cam plug had fallen out of… gone. In the search to pinpoint the leak, which was significant, 1-quart/15 minutes, I pressurized the system to 75 psi using a drill and the darn thing would not leak. This engine had to be running to leak. This test pretty much ruled out the gallery plugs. I suspect that a gallery plug would leak under pressure if it were running or not but the cam is effectively closing the
oil passage on the cam bearing when it is not running so the missing cam plug did not leak!
You can make this test, to help make a determination about the gallery plugs. Wipe every thing down in the area under suspicion and bring the engine up to temperature then pressurize the
oil system with a drill motor and watch for leaks. A gallery plug will leak.