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Firstly, I'm pretty sure the Dana 30 didn't come along until '67.. your '66 should still have the Dana/Spicer 27. There's a chance I'm wrong on that, but it's a pretty slim chance.
Secondly, indeed.. your car would have 4.56 gearing. But.. in overdrive, you should be looking at 19.7mph/1000 rpm (from Road and Track road test)... so, at 60, you should be at a bit over 3000rpm. At 80mph, you should be at a bit over 4000rpm.
I think your tach is wildly off.
You can check rear gearing by jacking up one wheel, and turn said wheel, while counting how many times the driveshaft goes around. One complete rotation of the wheel should turn the driveshaft 4.56 revolutions. You can also pull the diff cover and look at what's stamped into the outer diameter of the ring gear.
I think a '66 or older, non overdrive 122 *sedan* should have 4.10 gears. You could swap the guts, but the whole differential should be a bolt up swap. That would likely be easier than swapping the gears themselves.
4wd/off road type shops are generally pretty well versed in differentials, and should be quite familiar with the Dana 27 and Dana 30... both in available parts, as well as proper maintenance and gear swapping.
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-Matt I ♥ my ♂
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