Tuning the carb and timing might add a bit, but generally trying to eek out a few more MPG on a 1 ton van is a search for minimal gains.

I had a 79 B200 shorty van with a 318/833OD and 3.55 gears and it got 25mpg hwy all day long here in Colorado.
Also a 91 B300 with 360/518 and 3.55 and it got 21mpg.

Would love to know how that happens, my 02 gets 14.5 on a good day. I bought a 92 250 extended people van for the 360 518 to put in my LilRed but I might just stick the whole package in the 88 long bed 318 904 if I can get that kind of millage.

The best gas mileage I've ever gotten out of a Dodge truck, or van was from the V6 mini van years ago (which was nearly always near 20 mpg), and my current truck.
Last year my truck went from home here to the west side of St Louis (365 miles each way) The truck got 17.3 mpg on both the trip down, and the trip back up, and 14.6 while there (2 days). The sheet metal is sitting on a 96 Dakota 4x4 chassis with an EFI 318, and the OD auto trans, and 3:55 gears. The mileage on the interstate through Iowa was on the Mopar factory cruise control set at 72 mph. The gas stop on both directions was made at nearly the same mileage, from the same gas station. The truck was filled up 2 blocks from home, and the return trip was filled up 2 miles from the hotel we stayed at. All the gas stops were at the very same gas stations, and the mileage was recorded by the trucks odometer. The cruise speed matched the GPS speed. No bs here.

Pictures of the truck, probably the day before it went on the trip to ST Louis. Its a 49 Dodge pickup on a 96 Dakota 4x4 frame (yes, the 4x4 functions). The frame was unaltered except for the shortening of the wheel base by about 8" and uses all the Dakota mounting brackets, drive train, fuel tank, wiring and computer. Gene

Those 20 something mpg numbers were all rolling down hill in Colorado. No word on the uphill mileage.

That is a nice looking truck Gene. I had a 92 Dakota ext cab long bed 5.2 518 3.55 gear that would knock down 20mpg on the interstate. Tonneau cover, electric fan, and taller tires helped up the mileage a bit.

Those 20 something mpg numbers were all rolling down hill in Colorado. No word on the uphill mileage.

Gene, as to getting the seat back a couple of inches.
since you have the gas tank in the dakota location, and out of the cab, is there a couple of stiffening ribs on the back of the cab wall you could remove, allowing the seat to touch the back of the cab ?
just thinking out loud here, as i am not familiar with those year trucks.
or maybe you have already done that, and i either forgot about it, or missed it when you said you did that.

The top of the seats are under the back window supports. I can back the seat bottom up a bit more, but then the seat sits more upright and is a lot less comfortable. With my thick winter coat on, the wheel is very close to the coat, I have to suck it in to get the seat belt between the coat and the wheel. Its more of a fat old man problem. The truck also has a tilt column. If the column was a bit shorter, the issue would be gone, but then I'd have to change a bunch of other stuff. I'll just deal with it the way it is during the winter, without the heavy winter coat, there is not a problem. I would guess this old age thing is making me pretty picky, it wouldn't have bothered me in my younger years.

So should I put a build tread in the truck section, or the street rod section?

That's pretty much what I've been doing. I have a 2000 S10 truck that's my daily driver...
Here's the thing, I live in a rural area on a Ranch, it's basically an off-grid community and the county doesn't come back here. I'm thinking of using the van to start a business and provide some sort of service out here but I'm not sure what... Trash pickup, water delivery, basic home maintenance etc... I'd like to be able to squeeze out as many mpg's out of it to make my business more profitable.
I can't make much money if most of my earnings go to fuel costs... For example, it's about 70 miles round trip when I go into town...
In general it'd be nice to get more mpg's but not if I'm going to spend more on a swap than I would paying for gas...

As mentioned before in one of my other threads, I have a 94' B250 that's basically a parts van at this point. It has a 4speed auto A518 tranny and apparently it'll be possible to swap the tranny and get better mpg's but I just rebuilt the tranny in my B300!

Ok, say I optimize the 360 and rebuild the tranny out of the 250. What optimizations can I make to the engine and what mods are necessary to install the tranny from the 250??

Would it really be worth all the work to swap tranny's though? How many more mpg's can I get with the tranny from the 250?
I think maybe fixing up the 360 will be the most viable option considering all the comments...

For now, I guess I'll just focus on what can be done with the 360 to make it more efficient...
Apparently my van has or had the ability to run on propane?? There's a sticker under the hood that mentions the fuel mixture process for propane in that motor...
Any thoughts on this?