I think theres still some more constraints you need to provide and that we need to comprehend to really recommend a best-it.
if gas is truly no concern, then i vote RB 440-Stroker (500ci+) power, resale, bang-4-the-buck, ease of getting parts and fit in the install bay (maybe youll want to add A/C ?), as well as the amount of knowledge and prior experience out there on it.
moderate hwy driving Im not sure just how much youll need the 5 and 6 gears vs a 4 speed.
if you do a lot of HWY driving then, at min, a 5 speed
manuals are more fun to drive, but its easier to drink a big-gulp and eat cheeseburgers while drving an auto tranny. If heavy traffic is lots of stop and go, it gets stale working 1-2-1-2-3-2-3-2-1-2 every 1/4 mile or less.
another option is swapping in a 5.7 or 6.1Hemi.
