Tall gears and 4-speeds

AJ/FormS said
How does it drive?

Loudly. :( Engine seems perfectly happy to do it, but it's loud, it burns a lot of gas, and it cannot be good for engine longevity.

Can you be more specific?

About what? I'm not following you.

with your combo; 65= 2300 rpm which cruises fantastic

70MPH actual (on GPS-speedo is considerably off) is 3000RPM. Usual highway speeds in this area are 70-75MPH.

But the starter gear of 2.66 x 2.76= 7.34, requires a pretty stout bottom-end. It will require a good bit of slippage to take off briskly. I solved that by installing alloy heads and running the cylinder pressure up to 180 or better. My 360LA then had plenty of bottom-end punch.

You can get a leg up on it by installing the Commando Gear-set which has a 3.09 low. This would make your starter gear, 2.76 x 3.09=8.44, about the same as 3.23 x 2.66=8.59. The 5.9 IIRC comes with an Scr of 9.0 or 9.2 I forget which, but the end result is the same; a mild cam won't kill the bottom end.

I have driven tall-geared cars, including a Volare with what I think was 2.94 or 2.76 and 4-speed. Yes, I know that was a 3.09 low 833OD, but it was also a heavier car than my zero-option Duster, with a slant six. FWIW, the engine willingly pulls from 1000RPM even in 4th gear. Cam is a Hughes SER1418ALN10.

But as someone said; your Duster with 3.55s, and 28" tires would cruise at ~65= 2770 compared to the current 2980 with 26s.

but here's the thing; Second gear is 1.92 and with 2.76s, that will get you 44mph at 3000/ 60@4100, compared to your current combo doing 34@3000/ 60=5300, so you can expect your zero to 60 ET to be down a bit. Maybe quite a bit, lol.
I didn't run those gears for long because of that very reason. Eventually I bought a GVod, and now 65=2240 with 3.55s ............

Perhaps, for you, the A833od might be a good solution. With your current 3.55s and 2.76s; it will give you 65=2180, with a starter of 3.09 x 3.55=10.97/same as I run . But Second is a lil far away in it.
here are the ratios; first yours
2.66-1.92-1.40-1.00, next the Commando
3.09-1.92-1.40-1.00, then the overdrive
3.09-1.67-1.00-.73od
Road gears next; first yours with the 3.55s
9.44-6.82-4.97-3.55, (65=2980); followed by what they would be with 2.76s
7.34-5.30-3.86-2.76, (65=2320); then the overdrive with 3.55s
10.97-5.93-3.55-2.59od (65=2180); And then the Commando with 3.55s to 3.23s, then 2.94s
10.97-6.82-4.97-3.55, (65=2980)
9.98-6.20-4.52-3.23, (65=2700)
9.08-5.64-4.12-2.94, (65=2470)

IMO,
and for a 360/5.9 street car;
Second gear ratios in the hi 6s are pretty good
Second gears in the low 5s are pretty tame.
Second gears from about 5.5 to 6.5 are a good compromise to get a decent cruising gear.
BTW;
those overdrive transmissions can be had pretty affordably. But they do require a fairly wide powerband, which the modest compression 5.9 Magnum should have. IMO I would certainly try one before I would go to 2.76s.

The OD is an option...but that's a much bigger cost than a gear swap. (Especially since I think I found a set of 2.71 gears for $100.) Also a consideration: the trans in it is a fresh (<5K mile) rebuild from Brewer.