L31MaxExpress
I'm Awesome
I do not see how a LS would get better mileage. Never once have I seen it in the real world, the 5.3Ls tend to drink gas in a heavier vehicle and towing. Towing heavier or even just driving around the 6.0L 2500/3500 NBS trucks are thirstier than the same truck with an 8.1L.I put a 383 in mine a couple of years ago. And there is no comparison between the two. The stock Vortec fuel injection system is capable of supporting about 400 HP, so I had my stock engine built for 450. Brand new, a Vortec 350 was good for 250 HP. If somebody's telling you that 400+ horses isn't a dramatic improvement over 250, they don't know what they're talking about. I do not do any towing, but the thing hauls ass now. And only lost a couple miles to the gallon as far as fuel mileage goes. The only downside was I did have to replace the transmission and the rear end afterwards (they were just old and tired and couldn't handle that much engine). But I still don't regret going with the 383. An LS conversion would get better mileage though.
My 383 has gotten the best of 20.6 mpg running 70-75 mph in a 7,000 lbs van. The 383 does not have to rev to move the weight. Even towing 8,000 lbs up a slight grade it is in overdrive. Towing a lighter ~3,000 lbs full loaded utility trailer, I can keep it under 2,500 rpm, still accelerate well, get to speed and roll 80 mph without once getting over 2,500 rpm.
The 383 big gain is not so much the HP. I can build a 350 to make as much hp as my 383, but the 350 is not coming close to the low-speed torque.
I also had a L31 in a factory Mexican NBS Tahoe with a factory 5spd in it. That Tahoe was better on fuel than the 5.3/4L60E equivalent ever thought about being.