My 408 was approaching 600hp n/a, when I pulled it to freshen it up the main and cam bearings showed signs of block flex. That's why I bought the R1 engine. If I were going to make big power with a production block I'd do a short fill in the block to try to stabilize the cylinders. Plus front and mid plates.
This and $2.50 will buy you a cup of coffee in some places.