How to get 400 hp out of 1971 340 .30 over

The one place I’d argue is that, where do you find a stock as delivered 340 from ‘71 & down with the high compression pistons till running around today in good shape?

I think it’s a load of bologna because (IMO!) they all need to be rebuilt.
I have one :)
68 340 orig rings bearings
refreshed top end weiand stealth 650dp unilite 218 218 454 110 in 106 3.91sg tf-2 727
102k on the bottom end 169 psi cranking compression