400 block (hope to find a 230),If not whats the next best casting and year for a 400? 6pack rods,stock 440 steel crank, lightweight pistons ross or srp. How much could this handle? I have a set of indy cnc ez's and solid roller I want to put on.

They are all pretty much the same after the 230 blocks , you might try the 74 up block for better casting quality , less chance of bad core shift that plagued the early blocks , EVEN the 230 blocks had the chance of some pretty bad core shift .

I would be more concerned with those boat anchor rods than the non 230 400 block . HEAVY rod wit htoo small boat spining hi RPM is a recipe for DESTRUCTION .

I seem to recall that 600 hp is the "magic number" where stock blocks start to give up... use main studs and a good girdle to hold it all together.
But as already mentioned, check your cylinder wall thicknesses for core shift before putting money into a stock block

-Charles

I would not use the six pak rods, why not offset grind the rod throws to BB chevy journal size and increase the stroke to 3.90 or 3.91? That stroke makes a lot of power and likes to RPM, I would use a 6.7 H beam rod and have the pistons made for that combination

600 is probably right. both 451's i've built were 650hp motors & both died from block failures after a few seasons abuse. i didn't have a girdle in those days. best i could muster was nodular main caps with a stud kit. everything else was either the stock 440 ground crank or a source crank with eagle rods & SR heads. the blocks weren't the 230's but were sonic checked & deemed to be good. these days i use a Maxx block.