This was so significant that I copied it from the ClubCobra forum.  Many thanks to Mike!

Nearly all 427 center oiler blocks have a small (1/4") hole under the oil filter adapter pad that should be plugged. I'm not sure what the hole was for but I believe it was for some drainback to the pan when FEs were used in undustrial applications. Normally, when using a passenger car adapter the hole is covered by the adapter and gasket. But in Cobra applications where a remote oil filter and/or cooler is used, nearly all of the adapters have an open area over the hole that is only covered by the gasket. If the gasket blows out or leaks, oil pressure will bypass internally right back into the pan. Depending on how bad the tear in the gasket is, this will result in low or no oil pressure at idle when warm. I'm not saying this is your problem, but I have seen it happen plenty of times and it is easy enough to check out and fix. See photo, an allen plug is installed in the hole.

HTH,

Mike