Change gear ratio on 454 class C

It's possible and the coin isn't that bad for something in that age group. Whether it will help with fuel economy, who is to say. Heavy duty vehicles of that era are not required to report fuel economy estimates. Based on a quick look at the chevy brochure from 2002 for the cutaway chassis, chevy put 3.73's in the 12000 gvr, 4.10's in 15000 gvr, and 4.56's in 17000 gvr commercial cargo vans. So if the ready to go weight of your '98 is way less than the gvwr, it could probably pull 3.73 gears.

Next thing to figure out is how long it will take that 1-2 mile per gallon improvement to pay for the gear replacement.