What's the absolute maximum range of your Duster?

I once drove my Barracuda all day on a tankful, (67 liters, or 11.8 gallons US) at speeds up to 85mph.
On the way back, I followed a late-model Corvette who was caroosing, so didn't get quite the mileage. She was geared 85=2100

The factory long-stroke slanty can do very well cruising at low rpm, from point to point. But as soon as it gets into stop and go driving, it all falls apart. With hiway gears she just can't get the ignition timing she needs.
If it was me, I'd be installing an adjustable, stand-alone, dash-mounted, dial-back timing box; and tune it as you drive. Not for vacuum, but for highest speed at smallest throttle opening. There is NO OTHER Way to get the ignition timing right.
Let's say;
With 2.76s and 27.7" tires, 65=2250 at zero-slip. She could be wanting over 50 degrees. But your factory D will be lucky to make 20 plus your initial. Factory timing is around TDC so 20 it is. If you advance the unmodded D to 12*, you might then get 32. But your power-timing will probably drive the slanty into detonation pretty easily. It's a long long way from 32 to 50, never mind from 20 to 50, and in either case, it will suck gas.
The device is easy to use;
Just drive it up to your cruising speed on a flat level hard road, and wait for the SPEED to stabilize. Then dial in 3 degrees and wait. If the speed goes up,drop it back to your starting speed and repeat. Keep doing that until the speed no longer increases. If it eventually slows down, then back up 3 degrees.
If you run out of range, you will have to advance the D manually, and start over.
You will have to remember to dial it back down, whenever you change throttle setting, and especially if you have to gas it, or climb a hill. If you hear it rattle jump on the device! If it continues, back off the throttle until it stops. The device cost me about $170 back in the early 2000s. IDK what they cost today. Mine paid for itself very quickly. I even used it to dial in my power timing. And my PT (Part Throttle) timing. And I used it to figure out if carb changes were in the right direction.

Pump your tires up too.