Hello from Hawthorne Dave.
I had the 05 5.7 hemi in a 3/4 ton 4x4 with 33 inch tires. I couldn't get more than 15mpg at 70. With 4.56 37bfgs non o/d trans, in my 84 R/C and a old raggedy 360 I can barely get 70mph lol that's not entirely true. But my mileage at 70 and 3200 rpm is a dismal 8mpg if I slow to 55 its 14 mpg. I swear that the old 360s have more bottom end grunt but the hemi well its a rever 6000 6300rpm is where the computer would shift when carrying or pulling a load.
The 360s, magnum included just don't have the compression the hemi has nor do they have the gifted cylinder head. The cams that come in the magnum and the pre magnum just run out of efficiency at higher rpm. So in my mind the difficulty is keeping the bottom end grunt and getting some upper RPM efficiency with the 360s. Which I think can be gained in compression, cam and a head that flows well.
The Hemis also seem to be gifted with a better transmission, lower geared in first and quick shifting. It may have been the computer limiting torque but it just seemed revy not torquey.
This is all just food for thought, My mind doing what it does SQUIRREL. Good luck on your endeavor.