[Alex V.]

I think it stands to reason, all things being equal, a bigger engine producing the same power will exert less stress on the moving parts and therefore last longer. A 300 HP small block can certainly last a while, but a 300HP 454 is pretty calm and even the flat-tappet carb'd versions could rack up hundreds of thousands of miles towing loads.

Also, there's another particularly pertinent factor when talking about pulling: horsepower vs. torque. Let's compare numbers on a couple engines:

L31 Vortec 350, the standard issue light truck engine from '96 to 2000.
LE8 454, the extremely tame 80's carbureted truck big block.
LQ4, first version of the 6.0L (2000-2002?), with non-electronic throttle body.

Horsepower peak:
L31: 255 @ 4,600
LE8: 230 @ 3,800
LQ4: 300 @ 4,800

Torque peak:
L31: 330 @ 2,800
LE8: 360 @ 2,800
LQ4: 370 @ 4,000

So, the 6.0 is most powerful, and the 350 with fuel injection and higher compression on its side is darn close to the 454 with barely enough compression to combust the fuel and heads that a modern 2.0L turbo 4 cyl. would probably outflow. However! Notice the peaks. The 454 has a substantially lower horsepower peak than either of the others. I've not done a lot of pulling with a L31-equipped truck, but with the torque peak in the same spot as the 454 I'd expect it to not quite keep up, but perform admirably for an engine only roughly 3/4 the size of the 454. However, the 6.0's torque peak is extremely high compared to the others, which translates into the rev-happy-ness that others have mentioned about the LS engines. LS engines have plenty of power and they're tough - probably tougher than any of the engines before it when it comes down to it - but the power isn't down low where a sane person prefers to keep their engine while pulling. In two equivalent trucks, one with the carb'd 454 and one with a 6.0, towing the same load up a hill at the same speed then put to the floor partway up for a race to the crest, the 6.0 might edge ahead of the 454... but it has to run nearly wide open to do it where the 454 stays down where it was to start with and just pulls. To add to it, all these comparisons are with one of the most anemic 454's ever made - the Vortec 7400 puts out 290 HP and 410 lb.-ft. of torque @ 3,200, which is right about where it's going to be when a 4L80E shifts out of OD into third at highway speed. In this case, in our hill climb scenario, to even get all of the 6.0's torque you have to forget about third and go to second at which point you're just screaming and giving 'er all the air and fuel she can take so *whoop*, there goes your gas mileage. So yes, a Vortec 454 should rightfully run plum off from a 6.0.

Just a couple anecdotes: Last week I'd done some work on my FIL's '03 1500HD (6.0/4L80E CCSB) and drove it back to his place 35 miles away towing his 18' open car trailer I hauled it to my shop on... behind my '85. The trailer weighs 1,800 lbs., I installed a reman transmission 2 years/60K ago, and the 6.0 runs well enough I don't think it's lacking much of its original power.
So I'm climbing a fairly steep, straight hill at about 65 and gaining on a Tacoma that had just crested the hill only doing about 55, and I stepped into it (downshifted 4-3) to get past him before the passing lane closed back down to 2 lanes... but the genius in the Taco gained about 5 MPH once he got back on flatter ground, with me coming up on him pretty hard, so it was either flat out put it to the floor (and probably get a 3-2 downshift) or abandon my plan and stay behind him. I chose the latter, and was honestly very unimpressed that I was basically using all the truck's power to only gradually accelerate up what was then a slight hill. Err, well, I say "all the trucks power", but looking at these power curves I needed to get it into second to really get all its power... but with 300K on the truck I'd rather not grenade it.

Secondly, my brother-in-law's dad has a '98 K2500 Suburban with a Vortec 454 which has spent a good bit of time towing a 32' enclosed trailer (loaded real heavy at times) and has 280K on it. They just towed it from central KY, to central MO, to Wisconsin and back, running into a stiff head wind on the way to Missouri. He's also been known to set the cruise control at over 70 MPH with the trailer and let it downshift to 2nd on its own, on hills - which I can't even imagine doing, but the point is that 454 has done some serious work and it seems un phased by it. BTW, he runs Schaeffer's 7000 10w30 in it, as I do in mine.