Same thing for me with my 83 Cutlass. Factory v6 (w/ only 110 hp) car, got barely 16 mpg on highway... Drop in 307 (140 hp), same gear ratio, and I get 19-20 mpg on the highway. Now got 455 rocket (270 hp/390 ft lbs torque) with a 2004r overdrive, and on the highway I get 25 mpg at speeds upwards of 70mph. Between 55-65 I get 21-22 mpg. And this is an automatic.
455?!? Taking the big blocks out now huh? LOL. I had one of the last 302s in a 95, but had it bored .40 and stroked 1/2". After everything was said and done, only thing left from the old engine was the block. Mind you, I'm not getting 25 mph by any means but with the stock gear I still get 15-16 mph around 70 and 12-13 mph at 55.