I would try a programmer that mods the parameters that cause the truck to downshift. Is it decreasing speed when it shifts down, or does it just drop down when still maintaining?
I was looking into a gear swap for my truck with the 5.7 because I figured I couldn't really gain anymore power than a Hemi and keep a fair gas mpg. 3.55 to 3.92 would have been about 200rpm diff for me, and i don't think that would fix the problem I have. I was just looking to keep the 4 banger mode in longer while driving on highways with very mild rolling hills(very mild).
But after much research, the issue I think is ultimately going to get me is I like to set the cruise on about 75 and there is too much wind resistance on the truck at that speed for the motor to overcome in 4 mode without going up about 400 rpm and that would kill the mpg, and there is no program avail to adjust the MDS, which I have found. Tried the test and took the truck down to 63mph in cruise (very hard to get passed by big rigs
and it worked like a champ without coming out of 4 mode with the decrease in wind resistance the little bit of throttle input was fine to maintain the mode.
Hopefully you can find a programmer that mods the tranny and adjust things out the way you like them.