Like most things, the rear gear really depends on the combination.
My Hemi 4-speed RR had 4.10s & 28" tire, and with a fairly big cam/headers, it was happy at 3500 or so on the highway.
Get into the throttle, or for more fun drop down a gear & get the 2nd carb involved, and it'd rocket to triple digits.
Ate a lot of fuel but that wasn't a concern. Seemed it needed at least the 4.10s for low-speed driving.
One of my basically-stock 383/4-speed RRs had 3.91s when I got it, but seemed too busy; 3.23s calmed that down.
What it seems to want is more ratio at lower speeds, though.
For me, if applied to those cars, the Passon 5-speed is appealing for the ratios, but the price, delivery & issues spoil that option.
Added a GV to my 454 Dually with 4.10s many years ago, and it made the truck much better on the highway, driving or towing, and it's been bulletproof.
At 65-70 MPH, it's right in its peak torque - light throttle @ 2500-2800 RPM is a sweet spot - and the MPG is better by 30% than without the GV.
My Mustang (4.6/5-speed) with 2.73s has a nice ratio spread, and is great on the Interstate - 80MPH is roundabout 2000 RPM or so, 30 MPG with AC on.
That's what it's best at, so I've resisted the urge to swap in more gear to crutch the relatively weak power - it's the Interstate tool.
As far as power capacity, a lot of the Drag Week guys are using the GV.
Bottom line, it's all about having the engine in the RPM range that's best for the combination & usage.