A Gen 6 502 w/an EFI set-up on it, tuned to perfection (aftermarket, of cour$e), headers, low restriction exhaust system, an OD manual trans, w/'appropriate' gearing out back, can get as much MPG as your 300 cid SBC buzzing at 4000-plus RPM at cruise, in a 7-ton vehicle.
When you encounter a grade or headwind (God forbid BOTH at once!), the 300 cid engine in the decidedly un-aerodynamic 14,000 pound van will be so overtaxed that it will not be able to make the grade/headwind w/o losing speed. Never mind that you'll have worn a hole in the floor from matting the throttle so much of the time. And forget trying to use a vacuum gage as an "economy" gage- it'll never see enough vacuum to get out of the red/yellow, unless it's idling or coasting downhill!
Through the years Chevy was driven- not by the promise of quick ET's- but by the requirements of their truck lines, to produce the larger and larger engines. If all they needed to do was to put long rods in an existing engine along w/whatever else you have in mind, don't you believe they would have done just that?
The OM's are in business to make money. The cost of developing a brand-new engine design (like the W-series or Mk IV) is in the many, many
millions of dollars. They'd never
spend that kind of money, were it not absolutely necessary.
Remember, the original BB "W" Chevy engine, the 348, was a truck engine first. The TD BBC's were truck engines to the core. Even the Dodge Viper V10 started its life as a truck engine. Early Hemi engines were used both in trucks and cars (and even industrial pumps, generators, etc.).
In ALL those apps, if the job could have been done w/a smaller, lighter, cheaper design, it surely would have. But the facts are what they are.
At least you have a good backup plan for the little mill- an S-10 w/that engine, one of AutoGear's Muncie's and a set of 4.56 gears- NOW you're talkin'!