The 300 is finiky about fuel and prefers premium octane due to its fairly long stroke and side draft manifold. Also check for water content in the fuel. The factory single barrel carb was a turd so you did good to replace it but being tuned for a V8 I'm thinking the power valve is the wrong size. Try installing a slightly larger power valve in the carb to eliminate pinging under load. If it pings all the time you may go up 2 sizes on the jets and adjust your timing to factory +2*.