gear ratios

Yeah Dodge Guy thats what I wanted know. So a 373 would at least be better

I can change gear ratios a lot cheaper than a transmission. If I can get 10-12 mpg. Ill be happy right now with 180,000 mile its getting 5 mpg.. Good for dump runs!

Go for the O/D trans. 3.54 gears will make it work too hard and the trans will run hot. You say it’s racing. What is the RPM at 60? If your at 2800-3300 that will be normal for a 3speed gas truck. My conversion van 5.9L with 3.90’s in 3rd gear would spin at 3300 rpm. Did that all the way to Glacier NP and back!

right now its racing at 60 its top peak power, no rpms just a big fuel gauge. Not to mention 16 225/75 tires

The engine will be a modified 7.4 or 454 with old style cylinder heads called 781.This was the worst 7.4 Chevrolet built coming in at 240 hp. About 2500 dollars in machine work gives me a much better engine

Have you driven it yet? What rpm are you running on the flats? How well does it hold speed going up 6% grade or into the wind?

I would aim for about 2500 rpm running a flat road. But depends on how the torque feels. You don't want to be downshifting too much.

375 hp, 400 torque tells me its a small block? That's pretty hot for an '88, more hp and torque than a new one. I wouldn't change the rear axle gearing, I'd add an overdrive either by going to a 700R4 built for towing or an add-on overdrive like a Gear Vendors.

What kind of engine mods do you have to make that much power?

Honestly you likely will not notice much difference. Either in pulling power or fuel economy. Another question, will or could you ever recover the cost of the gear change in increased fuel economy?