Emotionally, I'm all for an L6, but towing shortens my temper quite a bit (try to pull out and pass on a grade).
If the gearing is tall and transmission ratios wide, a 292 may very well have better torque than a V8 - at the RPM range you're in (that's where the comparison has some validity: the peak torque per inch isn't too much different, but the "mildness" of the L6 leaves it down where you can use it without down-shifting). My 1/2 ton Chevy 292 3-on-the-tree amazed me at how well it felt in traffic - no need for revs, just ease the clutch out and press down.
The more power you try to get out of anything by any method (except high compression or displacement increase) moves the power away from your cruising RPM, and as it gets hotter the low speed power is significantly worse than stock.
A truck is tall enough to accept a 4200 (very tall engine - too much for most cars), but it's about 275 hp, and they're cheap enough but the EFI and spark have to be bought ($$$) or fabricated. They're also ugly as sin.
There are no car-size (under 800 lbs.) L6 engines out there bigger than the Ford 300, so displacement is always a problem.