I'm thinking a sort of homebrew Herkimer Battle Jitney.
RE: lowered vs. dams and skirts. For my part I would go with a combination. Extreme lowering would require a lot of jiggering of the suspension; just take it as far as is reasonable and doesn't job the suspension geometry, then do the rest with skirts.
You asked earlier about adding a fairing to the axle tube. That reminded me of a snap-on fairing for ultralight aircraft brace tubes. I thought it was ridiculous until the testers reported an extra 10mph or so with the fairings on - no other changes to the plane at all, just fairing all the exposed tubes that were crossways to the wind. Huge difference, and a lot less vibration in the airframe too. If there's still a fair amount of airspace above the axle tube then I'd say fair it; if not then include it in a belly pan.
I would be inclined to keep that massive V6. The GMC V6 is a lifetime investment - you won't live long enough to wear it out. You'll have to break it deliberately.
I just got done reading about
a guy who did some work on a 305: Mallory Maglite distro, GM TBI, GM Munci SM420 + GV OD, 4.56 axle ratio. Best mileage: 18.xx. That's probably under best conditions, reports 12-14 average while towing, worst case 4x4, uphill, towing at 18K all up for 7.2mpg.
Not bad, considering.