Probably the 74 intake is a spread bore (some of the 2 secondary holes are outside the "square"). You would need the thin flat adapter plate (it's cheap) to use a square bore carb (avs/afb/most holleys), which the plate covers up the parts of the sec bores in the intake that are outside the square. Use a gasket then the plate then a gasket then the carb. Yes it would help as a stockish eng will do better w a dual plane intake. If the carb is a '67 carter it'd be an AFB which is a far better performance carb than the '68-mid 71 AVS's which were all emission carbs but you can work w both of em with an eddy strip kit (just get the strip kit that has metering rods/jets close to the sizes in your OE carb) and if you do have an AVS just fill the domed metering rod caps w epoxy to (in effect) make them flat like (almost all) AFB's were as the eddy strip kits use 2 step rods. Some (not many) AFB's had 3 step rods w the required domed caps for 3 step rods but no AVS's had 2 step rods (w the attending flat caps)