[Zoomad75]

Just to throw a monkey wrench into the equation... My lifted K5 with a 5.3/700r4 combo rolling on 35" tires and 4.10 gearing with a camper on it average's 13mpg. I've seen 1 tank as high as 16, but the overall tank to tank average is around 13 and change. Without the aero drag of the camper I was consistently getting close to 18 on the highway if I kept my foot out of it.

You can't overcome the physics for sure, but actually this is where power can help. The typical smog era 305's and 350's barely eeked out maybe 150-180Hp on a good day back when they were new. Challenge that weakness with age, wear, lift kits and bigger tires without regearing and you are asking for single digit MPG's. My old 75 was just that, 350, q-jet, 4-speed (non-od) manual, 3.73 gears with a 4" lift and 35's. On a good day I'd get lucky and squeeze 10mpg, but I either needed a fat tailwind or run downhill to get it. Most of the time it was 7-8 mpg, with 4-5 mpg happening off road/low range running. The 35's did lend a little OD effect as the size negated the 3.73's to act like a set of 3.42's. So sure my highway rpm went down at 60, but getting up to 60 took a while and a right foot planted on the floor. It offset any savings I was getting at highway speed.

There is no doubt that LS swaps are expensive if you aren't doing the majority of the work yourself. But the modern tech, combined with an OD transmission can work wonders. My little 5.3 has almost double to factory rated HP of the old 350's (before my aftermarket control system came in and gives a little more) and it's that HP and increased torque that gets my fat rig up to speed easier with the OD cutting the rev's down to barely 2000 rpm at 65. Prior to loading the camper on the 5.3 had enough beans to spin those 35's too. Now I got too much traction (weight) to even try. But none the less it's operating so much more efficiently than the best carb of any kind tuned in as best as it can be can do. One other note about serviceability with the LS I'm not stuck going back to Holley or one of the other aftermarket EFI companies if something breaks. Outside of my control system everything is stock GM for a 2002 Silverado 1500. I can walk into any part store in the country and get what I need for it. That being said I've put close to 13,000 miles on my swap so far and outside of a fuel pump issue, I've yet to have any problems.

There is always the 12v Cummins route if you are handy with a welder and like your rig sounding like a Kenworth. Not having done one myself, I'd bet the cost of that swap makes the LS swap look cheap too.

Outside of buying a set of LS swap engine mount brackets (there are a ton of sources now), there is no real fabrication required to do it. Really comes down to electrical and fuel plumbing. Get the right harness kit and the wiring is simple down to a few connections.

If the LS is still beyond reach mechanically or financially, the new wave of self tuning 4bbl TBI EFI systems are the next option. The price is coming down rapidly with competition from many vendors. You still run your small block so no new/used engine costs involved. Most of the kits are comprehensive having most if not all of what is needed to get running. Still to get the most out of it, OD is still a requirement.

There is a lot of ways to skin the cat in this case, but without EFI and OD in these trucks you are going to be stuck to single digit economy.