Need to make a decision fast 5.7/6.1/340

I used to get 12-14 mpg hwy driving from my iron head 318/392 with 3.9 gears , 3400 stall, 727 and [email protected] flat tappet at 3000 rpm on the state limit. Motor used to make about 420 hp.

..move fwd 18 months - same car, exhaust, gears, trans, converter, 6.1 hemi, [email protected] roller cam, carb induction.....appx 450 hp

21 mpg hwy driving tested in March this year over 280 mile run at same cruise rpm on the same hwy