I had my Comp tappets restricted. The feed holes in the lifter were plugged. The side opposite the bushing feed was drilled .040". The problem was we used epoxy. Eventually the epoxy worked loose and slid back and forth in the feeds on some of the lifters. Never hurt anything. I think pressing a brass rod in there and then drilling might work. The funny thing is mine has huge clearance compared to yours, but has good oil pressure. Same lifters, block. maybe your lifter bushings are shorter uncovering a passage?
Doug