I am far from an expert.
That said, the first thing I would blame would be the oiling mods to the block you did.
I have never fooled with the factory oil delivery, and never had issues on 340, Or x block, running 7500 lots and lots of times, for years and years.
Fiddling with factory engineered stuff scares me.
You didn't do those mods to your old block, no failures. You did to this build, failure. That would be my suspect, to much oil somewhere, not enough some where else. I believe in your attention to detail regards measuring and assembly, that's proven good. Your other stuff ran a long time. Looks to what you did different....... Oil mods