I had a full size Chevy 15 passenger van with a 460 and got in the high 20s mpg. My son wrecked it, and I replaced it with a lighter duty and a 350. The smaller engine only got into the teens and the power curve was non-existent.
Bigger is often better and they’re are significant reasons to develop large vehicles. I never could figure why larger vehicles weren’t first on the hybrid path—trains proved the value of that technology the last century...
Your large vehicle would now likely be exempt from CAFE standards. “Large Trucks” etc. don’t have mpg stickers on them for that reason. They are not part of the equation.
How did you get a 460 in a Chevy? Only way would be a 383 stroker. Just saying, I’m pretty sure you meant a 454. The 460 was Fords big block.