Well, you're kind of between a rock and a hard place...wanting a rig that's rough and ready for serious trail work, but also good mileage on the road. Kinda hard to get both without some compromise. But....
...if you tweak the daylights out of something to get the best mileage, while also keeping it rough and ready, might, just maybe, get your a reasonable compromise. But, that all boils down to how you slice and dice things.
I can't comment much on the CFDs(Chevy/Ford/Dodges), but I do think highly of the 258 SJs.
Kinda biased, I am.
But even amont the 258s, you have a lot of variety, and options. If you go with low gearing in the axles, that'll chomp your MPG down considerably, but it'll give you better power. You can get a 258 with an auto or manual tranny. Auto, think slushbox......I'd only consider one for a DD if that's how it came. Plus, you'll get better crawl ratio with a manual tranny.
If you are looking for a '70s J-truck, then try to find one with a 258/T18/D20. Most came with 3.54 axles, but I forget if there were options in the J10s. You can always change that though. But the T18 with granny low will give you an awesome grunt mobile. I think my 78 with that combo has like a 40+ to 1 crawl ratio....and that's stock. I love the CJ folks trying to even get close to that with all kinds of "upgrades"
The T15s are nice trannies, but lack the granny low, so your grunt is less by default. The D20s also only gave you a 2:1 low range.
If you step into the 80s, you can get a 258 with a T176 and a NP case. The tranny is medium duty, and not as low geared, but the NP case has a better low range. Plus, some of these rigs came with 2.73 axles. Which helps on the highway, but not on the trail.
So, if you want heavy duty, you probably want to stick with a 79 or older, but you might have to compromise on the mpg angle some. If you want better mileage, but can live with medium duty (still quite capable though), then an 80+ might be worth considering.
Now, for eeking more mileage out of the rig, there are some things to consider, like TFI upgrades, running synthetics throughout, and some other things that have been discussed plenty on the forums here. I have heard of 80s+ rigs getting over 20mpg regularly. I have gotten a number of 19 and 20mpg in my 78, but I'd say 16-18 is a better "average", though I have by no means optimized this truck yet. So, 15mpg should not be a problem with a 258, and if you eek as much out of it as you can, no reason you can't be getting closer to 20mpg on the highway. Trail and city driving are other beasts.
But building it for heavier use, mudding, etc., you're likely going to eat the mpg some. But, prove us all a good deed and make the best of both. We'd love to see it. Afterall, yeh, anyone can do things with a C or F, and even a D, but just having a J-truck, even in stock form, usually turns some heads! Build it a bit and turn some more heads!
So, my vote - if you hadn't guessed it.... 258 J-truck.
You could go for a 360 J-truck as well, but you'll have to really optimize things to get above 15mpg, let alone anywhere near 20mpg with them.
I remember around the time Chevy came out with their new style trucks in '88, that the local ford dealers were running ads saying "don't you buy no ugly truck"....referring to the new Chevy style. It took me awhile to get used to those Chevies, but now I actually kinda like 'em. So when ford came out with their 96+ or whatever style, I couldn't believe it.....talk about ugly....sheesh. And I still am not overly fond of the 95+ dodges. My point.....them's all ugly trucks these days. Get yourself a good looking truck....a Jeep!
DON'T HACK YOUR WIDE TRACK FLARES!
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