Rover...if your '74 has vents BELOW the rub-rail, I'd bet THAT'S your problem...my '74 had same issues til I relocated vent to ABOVE the rub-rail...no more water in the gas causing the "stumble" you refer to... ;)...
You can see new location of vent here just aft of the cleat
this will SOLVE the "stumbles"... ;D...