you could be surprised by the fuel mileage on some, at least compared to todays trucks. I had a 68 Chev that got 12 mpg any time, trailer or not, and had a 400 smallblock in it. My 90 Chev that I had saw a best of 16... I had an 89 Ford 3/4 ext cab that got 12 best, 8 with an empty flatbed bumper pull... 351W 5 speed... and didn't even have that great of power to boot. In retrospect, I think the cat and muffler may have had some to do with that. It all depends what you want to do with the truck though, if the motor fits the bill or not. It also depends on the gears and how you drive it. I have heard of some of the 460's with higher gears getting into the teens back then. Tuning makes a big difference as well. I have seen cases where big motors in vehicles like that get better mileage than the smaller motors because they don't have to work so hard to get going, and keep going.