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The rpm range that matters is for highway. Pulling a load. That, out of OD it is cruising at 60-mph just below peak torque is the operating environment. (Miss this and everything else goes bye-bye)
Lots of guys tried your way in the 1970s. Not good results.
Take the van and get a TARE weight. Real numbers. Driver + max fuel. Then extrapolate our to axle limits. (A van is pretty much a 6,000-lb vehicle when used per design).
Same for tires. Measure the real thing as installed. Not numbers from factory brochure.
Tire rolling height is the first decision. A CRITICAL number if FE matters.
All the ducks have to line up.
I’d investigate what rear gear ratios were available in late 1970s vans. 3.55 and 3.73 were common.
Engine power comes from rpms. Not otherwise.
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