Consider 3 1/2" of ground clearance.
That should be a nice truck. It sounds like it's drivable now?*
I failed to find it but there was a hatchback car posted here that had a massive air dam with a notch in the center I wanted to use as an example. It was some theory like wheel spats and a jet of faster air down the middle to pull air out of the engine compartment or something. Also additional clearance so you don't high-center on gravel roads. Maybe aerohead has an example.
It looks like in your case, a bellmouth or plenum on the intercooler/trans cooler would be called for.
Have you thought about a varnished redwood 'barrelback' airocap?
https://www.google.com/search?q=chris+craft+barrel+back
Because I have.
Edit: *Okay, 13-second quarters. Got it.