That power level may very well have busted the block if it were an NA build. IIRC even 400 is dangerous.
One nice thing about primitive engines: with boost VE is far less important that modern stuff. As long as you can keep a gasket on it (and the crank in the block), 10 more psi makes up for any lack of port CFM. The ancient Studebaker V8 has produced some amazing results.