In the book "How to Build Big Inch Mopar Small Blocks" the author refers to a cup plug that must be installed in the block under the #5 main cap. I don't think there was one when I took the block apart ( it was virgin too) and my LA 318 doesn't have one there either (and it held good oil pressure at all RPM's). If I am understanding the book correctly, the galley in question dumps through the block in the oil filter base.

Help?

There are 2 horizontle oil passages from the oil filter boss in to the vertical passage from the pump to the main gallery. the oil goes up from the pump then must make a 90 deg turn out horizontally to the filter/get filtered then in horizontally to the vertical passage & up to the main gallery. The mini freeze plug is driven under the #5 cap until it is between the 2 horizontle passages to force the oil to go out/in to be filtered otherwise it'd go straight up from the #5 cap to the oil gallery and not make the turn out to get filtered. There is a recess/shoulder in the vertical passage between the 2 horizontle passages where the freeze plug stops when you drive it in easy and it is several inches down and you can see it (the shoulder) under the #5 cap to get an idea of how far to drive it in. EDIT if left out it'll have the same psi (in fact maybe a hair more) as the oil filter must have some restriction to do it's job but the oil will NOT be filtered