Domingo’s advice above is sound. I’m not a builder, but I’ve assembled a few over the years and I’ve never had a rear main leak. Both times my current 400/451 have gone together I’ve used the standard FelPro rear main with the rubber crank seals and pipe-cleaner & RTV side seals. The current assembly was 6 years ago. Not a drop of leakage.
I think the crank has a lot to do with it; I have no documentation, but believe the score-marks on the crank are an important component. If they’re worn out or your machinist disturbed them, could be trouble....