Are 400 small block chevys with cracks between the head bolt hole and steam pit holes use able?

You could try epoxy or tig welding, but you'd probably never get it to seal up. If it leaks water into the combustion chamber, you run the risk of hydraulic-ing the engine. Time to find a better block. Probably not too many 400's left in the junk yards, but the aftermarket is full of them.