Hello Tyler .
Yes you can extend the hopper and also build the hopper so you can put an extension on it to make it taller. With this said I would advise against it. I have had taller hoppers but only to cut them back down. .
The first issue is, sooner or later you will have a burp, belch or fart in the fuel hopper . The smaller the area of the hopper the less the explosion will be and maybe not even noticed . The more area the hopper has the bigger the boom will be.
But the main reason is the dakota has a small profile and will glide through the air using little power . If the hopper protrudes much above the cab more power will be needed.
Out on the open road the dakota should get about 100 miles on a loading of hard wood . The best that I have got is 122 miles pulling a small trailer.
While out on the road traveling I can think of very few times I made an unscheduled stop to refuel. 95% of the time while out travailing a fuel stop will fit in with a break.
The below pictures shows very little of the gasifier protruding above the cab .