Super Six Question

I just purchased a 74 Gold Duster with a Super six setup on the /6. It runs great, but milage is down a but it seems. I will go through all of the tune up procedures. I did notice a manifold leak and it would appear that the exhaust manifold does not have a real gasket. It would appear that RTV sealant was utilized instead by the previous owner.

What the hell!! Cmon, that is ridiculous.

Could this be negatively affecting milage significantly? I dropped from just below a half tank to below the 1/4 mark in about 35 miles of freeway driving at 65 mph. I havent had the car long enough to survey the numbers (only two days), but something doesnt seem kosher.