Diff ratio help

Most motors making 650 that I know of aren’t meant to run in low rpm ranges.

If the goal is low revs, highway cruising why not save all the hassle and out in more ‘simple’ combination. Even a mopar crate 392 or something, you can have EFI, power (what do they make these days 465 - 485? Can’t remember right now) and a five speed transmission so you don’t have to cut the hell out of the tunnel?