tuning a magnum

Put a temp fuel pressure gauge that you can read thru the windshield. Place a paperclip on the secondary vacuum rod pushed up against the housing. Run the car WOT and watch the fuel pressure gauge when it stumbles. If pressure drops there's your problem. Check the paperclip and see if it is push down on the rod indicating the secondary opened. Due yourself a favor and buy a wideband setup, there cheap and you can dial a carb in easily.
That carb is plenty for your motor. Vac Sec on street and DP for the track is what I prefer to run.