Here is a dyno sheet of a 572 inch Mopar big block with EZ heads and a 280/296 cam. This engine pulls past 7000 RPM and makes more than 860 hp. I think you have something funky going on with your engine. Maybe the valve springs were not correct for the cam? This engine uses the 325 big EZ heads which flow pretty well but I'd think a set of fully prepped SR heads would flow more air than EZ heads. The intakes should be roughly the same but the SR heads should have a better exhaust.