It depends on how much power you going to make,most people will tell you to buy the SHP block and stay away from the 400
the shp block is rated to 600hp mabe alittle more.The gm block maybe 600hp,but they tend to crack
my 400 gm 2 bolt block is on the engine stand with splade main caps,hard blok to the bottom of the water pump holes,light deck job and cleaned up the cyl's to .020 to make a 404.
at frist I wanted to stroke it ,but i will be conservative with it.