The only way I can think of to get 15mpg and 300+hp with that combination is nitrous oxide, but then of course you wouldn't have 300hp all the time, only for short bursts. Sacrifice the gas mileage - go for speed! I get 11mpg in my 351C 4sp 69 with mixed driving and a not-so-light foot.
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Neal
69 stang
351C/4sp
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