Lee, my experience with 454s parallels yours. I've had 2 454s, a pick-up & a Suburban, and a 455 olds in a GMC m.h.
8 mpg, all. Big difference was, with the 454s it was one tank of gas, one quart of oil. With the 455, 2 tanks of gas, one quart of oil. With the Suburban @ 75 mph, it was one tank of gas, 2 qts. of oil.

I've also had 3 Mopar 360s, 2 in van conversions, one in, my current S.O.B. Not nearly as predictable as the 454s. MPG has varied from 7.5 on S.O.B. with a toad, to 13 on one of the conversions.