As I understand it, a gear reduction starter helps primarily on higher compression engines. Am I correct?

If so, is there any advantage to using one when the existing starter (now 41 years old) is functioning properly?

Thanks.

No, unless it's in better condition, I suppose. If the current starter is functioning fine, there is no need to replace it.

The gear reduction starter is a bit lighter and will take less battery power to crank the car over, but otherwise, both will work fine. Only upgrade when the old one dies.