what am i doing wrong?

If you dyno'd your engine and it showed peak hp at 6300, your engine should have no problem running at that rpm. The peak is where everything is running optimum thats why it will start falling off past that. This is a normal condition. As long as all the parts you used to build the engine are rated for that rpm, you will not have a problem. In a basically stock 340, 6300rpm should not be a problem as long as the valve springs are matched to the cam you are using so you don't float the valves. If your peak hp was at 6300rpm and your engine builder told you not to spin it past 5900rpm, you should ask him why because if the engine was built right there shouldn't be a problem.