74 Duster 408 Daily Drivability with 3.73 Gears Vs Stock 3.21 Question.

The original 4-speed should have the T/A ratios. namely; 2.47-1.77-1.34-1.00.
This is a close ratio unit, sooooo you gotta watch your rpm Second gear is not too low.
With 3.23s and 27" tires, yur gunna get about 14mph per 1000 rpm so
2000rpm =28mph, and 32mph=2290.
If yur planning on installing a stock lo-compression 360 in front of that, the Wallace calculator predicts that factory 360 will have pressure near 133psi@730ft elevation, and feel about like a 318Magnum..... below about 3000rpm.
It's a good thing yur not looking for performance.
But, at that pressure, and geared at 65=2600 (3.23s); yur also not gunna get much for fuel-economy; even after you fix the cruise-timing.

If yur looking for max fuel economy, that factory 360 needs to be pumped up to near 200 psi, which cannot be run with iron heads and pumpgas. But can, with alloy heads. This will take about 10.8 Scr with the factory cam. and it will be a killer street-engine..........
Except for the rather modest factory cam, that's about what I run.

Be advised tho, that with 3.23s and a 2.47 first gear, the starter gear is just 7.98, and about the slowest you will be able to drive, without slipping the clutch, will be about 6mph@600rpm.... no Parading for you.

A better combo for you, might be the A833overdrive and say with 3.73s.
The ratios are 3.09-1.67-1.00-.73od.
With 3.73s the starter gear is 11.52 for 550rpm=3.8mph, now yur talking!and
65mph= 2200, which with a stock type cam is very cruisable, and you can almost give it max cruise timing. By 70/75 you'd be in.
As for second , 1.67 x 3.73= 6.23 and 32mph will be ~2480, and your 10.8 360 would easily motor away with that gear.

BTW-1
1.67 x 3.73=6.23 is the same Second Roadgear as as the standard Second gear, 1.92 x 3.23=6.24, so yur right in the Ballpark there.
BTW-2
I ran a combo like that, with 3.55s, with a 270/276/110 cam for several years. I liked it ok but when the cam dropped lobes, I went for the next bigger one. Bad idea, I've been waiting for this cam to die, since ~2005.

HappyHotRodding