I have the same issue with a Butler built 455 I inherited with a vehicle purchase. After 3 weeks of hair pulling, bought a black light and dye after replacing valve cover gaskets (leak) and oil wet breather and oil leak from stock pan. Turns out the genius installed the wrong dipstick and tube and filled it up with oil to the full line...

That full line was 9 quarts.

I'd confirm oil capacity, invest in a 12v blacklight, dump some dye in, and track down source. No or clogged pcv can cause blowby..but the pan should have a fingers clearance. lf not, likely the pan buckled and causing the issue.

Before I used tracker dye...I used purple stuff degreaser and pressure washed block with hot water.

Turns out the 3 quarts extra was seeping past the pan gasket.