I've had a lot of 360s, gas mileage is not one of their strong suits. My experience with a 360 is 2-3 mpg lower then a 318 in the same vehicle and the same drivetrain and fuel distribution system. I have never driven a 4.7, but the many people I've talked with tell me their gas mileage isn't as good as a 318 either. .

Next, that 56 Dodge pickup isn't exactly designed to slip through the air. I believe mid 20s mpg is a pipe dream with any motor in that truck unless you plan to keep it under 60 mph always. High teens, maybe 20 mpg is doable, much more then that I'd be surprised, and I don't think those numbers would come from a 360.

My 49 with the 5.2 Magnum, efi 46RE, with 3:55 gears, 235 x 75 x 15 tires, and a soft bed cover is getting near 17 mpg @ 70mph. It is a 4x4, so I would expect a 2wd to add a couple mpg at the same speed Drop the speed to 60 and the mpg jumps up.