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Garden variety 390s in sedans were not real hot motors. Even in a '67 Mustang, they were good for a 15+ second quarter. A bit out of the running compared to other similarly sized engines of the era. Pickup motors are even milder. They were intentionally made with sunken pistons for low, low compression, making them less powerful than the car versions, thirsty, but reliable under relentless heavy loads.
BUT, if you add a set of headers and fiddle with the induction on a car spec 390, things start to get interesting. (FE exhaust manifolds are possibly the worst ever installed on a V8) Add a bit of cam, and the 1 HP per cube numbers come up pretty easy.