I have a 74 Ramcharger that's a factory 440 truck and in the last few years I've driven it over 800 miles. It's fun, loud, all stock except for suspension with 3.54 gearing in the D60/D70 axles with 33" tires. I'm averaging 6.8mpg. If I was to build an older dodge truck to tow with, I'd use a 360 magnum block and make it a 408 stroker.
Those 33" tires are what is killing your mpg. It takes a lot of throttle to get them rolling. Then, has your speedo been calibrated to the big tires?
So is a 408 stroker motor any more fuel efficient then a 440 with a good towing cam?
I've pulled a car trailer with a factory 440, and I've pulled that same trailer with a 360 in a truck, the 360 did OK, but didn't hold a candle to the 440 truck, and both got the same gas mileage pulling that trailer (about 9mpg).
I've never had a 408 Stroker, so I have no comparison between it and a 440.
