LONG story, but one that I don't mind retelling! This was a big project that caused some at home.
When the old 3F engine gave up the ghostlast April, I took time to think about this and what I wanted to do. My initial reaction was to source another 2F in good shape to just throw in there, but since I can't do it myself and would have had to farm it out regadless, I didn't want to go through the expense of an engine replacement to end up another under-powered donk yielding 11 MPG. I also considered a V8, for a long time, but again, the thought of going with a swap that wouldn't yield a decent mileage was also a tall order, especially since the price of fuel kept going up up steadily while I debated the issue. So as I got closer to making a decision, diesel really looked like the right option, especially since I hope to start getting back into long-range trips.
I initially considered a Toyota diesel and if money wasn't an obstacle, would have chosen a 1HD-FT, but for someone already farming out the engine swap, $8K for a motor was out of the question and the hidden costs of a Toyota OEM diesel swap are all the little bits and pieces that you need to order from Toyota and that nickle-and-dime you to death. Besides, I know a lot of about running a non-US Land Cruiser as a daily driver (I spent the better part of ten years doing it), and it's not as easy as the diehards make it sound like, it can get UGLY when you need something that has to come from Japan.
So that's how I came to the Cummins 4BT. I originally wanted an Isuzu 4BD-T, but nobody seemed really familiar with the conversion and I didn't want to be the guinea pig, not when Proffitt's Cruisers was already well on its way to turning the 4BT conversion into essentially a kit conversion.
The 4BT weighs roughly 100 lbs. more than the original 3F and the Marlin Toybox weighs another 40 lbs or so. Add the extra 15 gallons of fuel and whatever the new tank weighs over the original. On a truck this size, the weight difference is negligible, and I saved about 80 lbs. going to the smaller sliders (the previous ones were a pretty poor design and had to be grossly reinforced to compensate for it).
I've put about 2,000 miles on the engine since the conversion, most of it on the jaunt from Boulder, CO to Austin, TX last week. I haven't cycled a full tank through the motor at this altitude (It's still got about 1/4 tank from the last fillup in the northern outskirts of Oklahoma City), but the fuel economy was a tad under 24 MPG on the drive from Colorado and that was at a constant speed of 70-80 MPH, and a fuel leak big enough to cover the entire back 1/3 of the truck with raw diesel (banjo fitting on the hard line was loose, I didn't have the right tools). I would imagine (and hope) that the fuel economy around Austin, where the highway speeds will be 55-70, should be even better. I will also do a dry run with the roof rack on soon, just to see what the variation is.
I estimate the expedition range (roof rack, trailer, loaded, dirt roads) to be somewhere in the vicinity of 760 miles or so. That means that with a full compliment of jerry cans (INTI racks are designed to carry seven across and two more on the trailer) I could theoretically go about 1,460 miles without refueling, but that's just theoretical, lots of variables impact that and besides, there aren't that many places in the WORLD where you would have to do that. I don't even like carrying jerry cans on the roof rack unless I absolutely have to, in fact, I got the bigger fuel tank so I wouldn't have to.