318 dodge dart stumbles 5000rpm

I'd put it on a fuel or ignition problem, with the symptoms really only appearing with a heavy throttle application I'd put it on the fuel side (although that could still be a timing advance issue). Without knowing more it could be rich or lean, but it sounds like either the carb tuning is off, or you're not getting enough fuel to the carb.

I'd check the fuel filter, float levels, and the size of the secondaries. The spark plug condition should tell you if you're rich or lean.