I admire your imagination but I think you're pushing at the wrong potentials.
I used to drive a 1990 Dodge B350 Wagon, the 15-passenger van, about a half-generation newer than yours. It never got worse than 10mpg, and it never got better than 15mpg. I was doing almost exclusively city miles, so never going below 10 was pretty good. Mine had TBI on the massive 360 V8 and a four-speed OD auto.
You didn't tell us what engine you have. For your sake I hope it's the 318 (5.2). The depth of knowledge from all sorts of gearheads for the 318 is enormous. The range of go-fast and go-long parts for the 318 is stunning. The flexibility of aftermarket modifications is fabulous.
You could convert your existing ride into a surprisingly thrifty cruiser. Believe it or not there is such a thing as a four-speed manual available for that van. Spend some time nailing down a good, reliable tune on the carb or convert it to a good TBI, perhaps with a lean burn mode, and I bet you could knock out 20mpg all day long at modest let's-go-see-America speeds.
If you are absolutely determined to go the hybrid route, don't mess with the drivetrain. There's a yawning abyss of space under the B-series, fab up racks and install batteries, and build a pusher trailer like
Mr. Sharkey. Bolt a rack of solar panels to the roof and the battery pack powers your occasional boost and also your comfort loads, lighting and whatnot.
NOTE: Mr. Sharkey's pusher trailer was, in fact, a diesel VW Rabbit nose that would push his EV on long trips. In retrospect if you wanted to make your van into an EV and have a diesel for cruising, that's a concept worth exploring too. In any case you could aerodynamically couple the trailer to the van, and it would contribute a net gain on your fuel economy even when it wasn't pushing, in addition to allowing you to move some storage out of the main vehicle.
The wood gas solution is a little wiggy in my opinion. I think those gasifiers are finicky and hard to manage. That's just me, your mileage may vary. But for the money you could spend building one and adapting your existing engine to it, you could just as easily make your existing engine into a really, really good modern engine that was affordable, supported in the existing infrastructure for fuel and parts, and wouldn't get you pulled over because it looked like your trailer was on fire.