Cool thread, i'll just leave this here. This is my "beater" I got for $300 and mostly used parts from previous builds that got scrapped. It has a 460/4bbl, C6 and 35 spline D70. It's an F350 (from 73-79 F350 2wd were built on a 140" wheelbase, mainly to help with the massive slide-in houses that were en vogue at the time). Believe it or not, it gets around 12 mpg on the highway if you keep it cruising around 55-60. It also has more wheel travel than a Raptor
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These trucks are a lot lighter than you'd think, there is a lot less insulation and padding in the cab, and virtually no safety features. Not even collapsible steering columns usually. New trucks also have massively overbuilt fully boxed frames, old trucks are all C-channel and have about half the cross-members. My company owns a 2012 CCLB and 2014 CCSB Ram, both trucks are WAAAAY over 8000lbs empty, my truck is ~4500 with me and ~22 gallons of fuel, and its still rated for 9900lbs GVW....These old Ford trucks are tough as nails and don't have a lot of the major issues that the similar year GM and Dodge trucks have, and if you look under the front of a 2wd Super Duty, you'll see that not much has changed over the years. Beam Team FOR THE WIN!! One thing to look out for, the F350 frames were quite a bit beefier than the F250 frames, they have a much taller vertical profile, and its almost twice as tall as an F100/150 frame at the thickest point. The 2wd F350's also have the biggest brakes, cold forged plate radius arms, and 1" king pins. A lot of the pre-runner guys used to rob beams and arms from F350s because they are so heavy duty but still interchange with the lighter trucks. Also, a lot of the parts for the unique F350 bed are long out of production, including the rear gas tank and parts for that odd ball spare tire carrier.
You're on the right track with a modern engine, you would see 15 pretty easily with anything under 6 liters of displacement, overdrive and EFI in my experience (although tire choice and driving style still play a HUGE factor). The main question is how much of a load are you needing the truck for?
That's about 5000lbs of concrete, and the 460 didn't even care or heat up (it was over 90* that morning). There really is no replacement for displacement when it comes down to moving things down the road, those coffee can pistons can do some serious work especially at the lower end of the RPM range. They really do get 10-12 mpg loaded, unloaded, up or downhill, tail or head wind, running or not lol. It just comes down to priorities, do you want a big rattley school bus engines that wakes up everyone in a 4 block radius but can tow a house behind you, do you want a simple old carbed big block that can pass everything but a gas station and be rebuilt in a Napa parking lot, or do you want a modern EFI small block with half the displacement and twice the RPM range, along with all the modern wiring needed to make it work? The upside? Its a super simple, MASSIVE engine bay that will accept damn near anything. 4v53 2 stroke detroit? twin turbo 572 ford? 12/24v? yup, seen them all crammed into one.
And just to stir the pot, it has been my personal experience that most people are exaggerating about what fuel mileage their diesel gets; back to back test I've done with the same 6k lb crawler on a trailer behind both a 8.1L GMC (my dad's) and Duramax (my best bud's) 2500HD trucks revealed that the D-max got about 2 mpg better overall. Hardly worth the hassle of oil and coolant changes that take twice as much fluid, (generally) more expensive and harder to find fuel, lots of extra weight over the front end leading to decreased component life, and the more expensive maintenance/upfront costs of the diesel. The 8.1 truck averages 16-17 highway, and the D-max averages 19-20 highway empty. That same crawler on a trailer behind a tuned and straight piped 7.3 power stroke netted around 13 mpg; again, only about 2 better than the 8.1 GMC got towing the same setup. Interestingly enough, our 2014 Dodge towing a similar jeep on a trailer, only manages about 10-11 mpg with the fancy schmancy 6.7 Cummins (stick or auto, we have both and the mileage is the same). I'm generally of the notion that no matter how you are trying to move whatever it is you're moving, it'll take about the same amount of energy. Just pick which sound you like best and roll with it!
