Ok, now I'm confused. Didn't you say you wanted to make your 350 into a 383? If so you will need a 400 crank.

If you are asking whether stock 350 cast pistons and a cast crank will take 6K rpm. Yes they will. Stock car claimer motors were all built with cast internals.

Most of us have wanged cast pistons way farther than the factory ever intended.

I have blown up my share of engines but I have never broken a cast crank. I have spit stock rods out the side of blocks, burned holes in cast pistons, but never have seen a cast crank "broken".