I suppose it might depend on the engine, the torque converter, and the amount of time one will spend at 70-75 mph.

A long stroke Pontiac engine, with a reasonable cam, can make enuff torque to drive down the road at 1600 rpm.

But, if you have a converter with enuff stall speed to slip a lot at 1600 rpm, it can build up extra heat, and possibly reduce converter life.

But, just because the trans has an OD high gear don't mean you'll have to run it in high gear when your engine is only turning 1600 rpm. You can just run it in 3rd gear til your highway speed is high enuff to need the OD. I suppose you could just think of high gear as something you only use when cruising the freeway, at 70 mph or faster.

The only reason I'd consider changing to a numerically higher gear ratio is if I wanted more acceleration in 1st, 2nd & 3rd gears.

Just curious. Are there gear sets readily available for the '65 Cat ? Don't know what rear end they had. I think '64 was the last year for the 9.3 chunk type rear end. Don't think they used the 8.2 BOP, like the A-bodies used. I read where 1 guy found some better gears for his big car, out of a Pontiac hearse.