I'm pretty much fine with my 3.7, only time it's really had trouble was when I went to Colorado in November, going east over Vail pass it really, really struggled towards the top. It didn't help that it was freaking me out watching them temp gauge climb a little then crash down as the thermostat opened (it was maybe 10 degrees out so the coolant in the radiator was getting cooled too quickly thanks to the speed and the clutch fan), it kept doing that several times before I eased up because I was worried the temperature shock might cause something to give up. I just had to slow down a bit to finish making it over the top. And that was with a Thor bumper, a HF 12K winch, my custom UCP that weighs maybe 75-90 lbs, me, my wife, and our three daughters.
Anyone that has gone over Vail pass knows it is STEEP, and kinda long. We didn't go back west that way though, we went up through Wyoming instead coming back home to Utah. Everywhere else though on that trip it did fine for a heavy vehicle with passengers and gear. If you want to talk about slow, try driving a UHaul truck with all of your crap in it. We did that moving from Colorado Springs to Centerville, Utah and it would just bog down so bad going up the hills on 80.
Honestly, for the money you are talking, if you really want a V8 I would just do a swap. There comes a point when you've modded a vehicle so much that starting over just isn't monetarily feasible, which it sounds like you are at. If you do the work yourself a swap isn't all that much money really. An alternative is to do like 4speed and do a SAS and gear it better. Heck, he's got a pallet ready to go for $4500 (which I would be all over if I had $4500 sitting around). Top speed isn't a big deal in our vehicles (I don't know about you but the though of going even 100MPH in the H3 is terrifying, if I can at least reasonably hit 85 that is all I need anywhere in the USA) so the higher(lower? never can get that straight in my head) gearing will just make it better to drive.
All in all, the 3.7 is fine I think, even with a bunch of crap on/in the truck. I didn't buy the H3 to drag race it. That's what muscle cars and hot hatchbacks and European performance cars are for. Instead of needing to out accelerate everyone, I can instead push my way in with my big mean looking H3. Or run them over. Take your pick.