Small Block Mopar Lifter Failure

Apparently, putting magnum heads on a Mopar small block is problematic. Perhaps it's that dark cloud that follows me, but three sets of lifters failed. Then a modification was done to the third lifter, and it started pumping oil. But wait... there's more: now that the engine has been built a second time, I have very low oil pressure at the gauge, and the engine might have to come back apart. Again. **I guess I need to add to this since a couple people want to point fingers and make sarcastic remarks: I am NOT the engine builder. Both failed builds were done by two different reputable (?) Mopar shops that said they could build this engine. Both builders knew what parts were being used and both started with a bare block, fresh from the machine shop. Neither could accomplish the build.