Intake manifold for 225 slant 6

On my 1980 Volare automatic,I noticed no difference with the 2bbl, vs the single, when using the original single exhaust. No extra power, no extra torque, not even better mileage.
I kindof think this is an all or nothing swap.
I was just looking for better fuel mileage. At least it wasn't any worse.
Good luck.