I have that van with a 305 in it. I don't know what it was getting before I bought it but I replaced all the ignition stuff, spark plugs, wires etc and put the tallest tires I could get on the rear. I get 18-19 highway. It is an automatic.
I had a 1960 Ford Falcon van that got 14 mpg when I was in MT decades ago. I put a 289 from a Maverick in it and got taller rims and the tallest tires all around. It got 27 highway fully loaded with gear and four climbers. It was a 3 speed manual.
If I could find a manual transmission for this GMC I would consider it. But they also have a major transmission cooling system that would have to be plugged off.
Detroit has no interest in producing high mpg cars. It isn't rocket science. You can do it in your back yard. Just get the right oomph to gear ratio.