Change gear ratio on 454 class C

How much power "loss" do you want to experience?
Depending whether its a mark V TBI 454 or a mark VI Vortec, you have between 230 and 290 hp and 385-410 ft lbs on tap. Light duty vehicles got the Vortec in '96, but idk if that crosses over to medium duty chassis applications, sometimes they run later than the light duty offerings, meaning you could still have the (even more) anemic TBI engine.
That's not alot to move a heavy 8x12 box down the road, either way.
What rpms are you running now at what speed in what gear? How long does it hold OD before dropping to 3rd gear in wind, hills, elevation?
Presume being a 98 it's a Vortec with a 4L80. Having an overdrive gear "compensates" for the lower final gears.

Bottom line, it would be a big mistake to gear it up much at all, much less that far up.

no it won,t work ,you should be getting about 8mpg. . drive 55mph you might get 9.p.s. I bought a sprinter diesel and get 20mpg mines a 5cyc.

Seems like an expensive way to improve gas mileage. On one of our trucks, an F250 we increased the rear axle ratio to improve towing performance, never mind gas mileage.

Maybe 3.73 but not 3.23. 3.23 will cause engine with the weight of MH will lug down.