Need some help, oiling issue 318 la block with magnum heads

The distributor side rear gallery plug effects the drivers side pressure most and the problem is reported on the passenger side; if that was the cause, it would likely cause oiling issues in all of the lifters. Still check it, and the front gallery plugs, but I'd be looking at other issues too

This may be a long shot but...... If the lifters have been swapped and the problem persists in specific positions, then it may be in the oil holes in the side of the gallery that provide oil to the lifters. The lifter bores are bored down so that they partially intersect the side of the oil galleries; the result is a small, oval shaped opening in the side of each lifter bore. This has to be big enough so that the 'waist band' (the recess around the body of the lifter) intersects the hole in the side of the lifter bore.

Check these holes in the sides of the problem lifter bores and see if they are smaller than the rest. Sometimes the stepped drilling process in the oil galleries will result in some size variation. Then measure how far down the waist band's lower edge is going down into the lifter bore and make sure it is getting down far enough to be exposed to the hole inside of the lifter bore to get pressurized oil, at least when the lifter is at the bottom of its travel.