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The pinnacle of slant six fuel economy was the 1976 Plymouth Feather Duster / Dodge Dart Lite, which was a lightened 2-door coupe (150+ lbs lower weight with some aluminum components) with low rear axle ratio (2.76:1 for automatics, 2.94:1 for 4-spd OD manuals), high-flow exhaust system, and a specially tuned 225 cubic inch slant six engine.  The EPA city/highway ratings of the Feather Duster / Dart Lite were 22/31 mpg (7.6 litres/100 km highway) with the automatic and 24/36 mpg (6.5 litres/100 km highway) with the aluminum case 4-speed OD manual transmission.and many owners of these cars with automatic transmissions report highway fuel economies of 31-32 mpg.  These cars also had EGR systems to control NOx, which also helped to reduce engine knock.  The Feather Duster distributor used the PN 3874795 & 3874796 distributors

From previous experience, I found that my highway fuel economy is best with a fuel mixture that is just rich enough to not have lean surge. It is generally accepted that the optimum total mechanical (initial + centrifugal) advance on slant sixes is 30-32°, all in by ~3000 RPM. Some have found that the best fuel economy is 44-45° of total advance (mechanical + vacuum) while others find that the best slant six fuel economy has 45°-50° of total advance. Since I had my initial timing set to 8° BTDC, I think there is still room to improve my highway fuel economy.  The old Scamp distributor (PN 3656859) had worn bushings that caused my timing to vary so I replaced it with a Direct Connection electronic distributor (PN P3690788).

Distributor
2444648
1965 Barracuda
3656859
1972 Scamp
3874795 & 3874796
1976 Feather Duster
P3690788
Direct Connection
Initial Timing BTDC 2.5° @ 550 RPM (D) TDC @ 750 RPM (N) 12°@ 750 RPM (N) 8° @ 600 RPM (D)
Advance Starts 1° @ 475 RPM 0.5 - 4° @ 550 RPM 1.5-3.5° @ 650 RPM  
Intermediate --- 8 - 10.5° @ 800 RPM --- ---
Full Centrifugal Advance 12° @ 2200 RPM 14° @ 2000 RPM 5.5° @ 2300 RPM 11° @ 800-1200 RPM
(single red spring)
Vacuum Advance Starts 5 - 7 " Hg 7" Hg 7" Hg  
Full Vacuum Advance 7° @ 15" Hg 8.5° @ 11.5" Hg 12° @ 8.5" hg 7° @ 13" Hg
PN 3755073 or 8755073?
Total Advance @ 2500 RPM 28.9° 23.9° 39.2° 44°
Maximum Total Advance 41.1° @ 4400 RPM 29.9° @ 4000 RPM 43° @ 4600 RPM 44° @ 2400 RPM

With the low-ratio axles, engine RPM at highway speeds would be nowhere near the 4000-4600 RPM centrifugal all-in.  With my car equipped with a 2.76:1 rear end and automatic transmission, 100 km/h (62 mph) translates to around 2500 RPM. With the Feather Duster distributor, I calculate that I would have about 39° of total advance at 2500 RPM and 43° of total advance at 4600 RPM & over 12" Hg of vacuum.  Even with the Feather Duster distributor, it looks like there are opportunities for improved fuel economy with a recurved distributor.

There were several options for vacuum advance but Standard Motor Products has discontinued many of their products so you might need to go to eBay to find a specific component.  Dan Stern provided much of the following list in Distributor vacuum can specs (list).  Since the VC-187 cross-references to both slant six and small blocks, it looks like Mopar vacuum advance canisters are generally interchangeable.

Mopar Vacuum Advance Canisters

Standard Motor Parts PN
Start
Maximum
VC-93 4.9" - 7" Hg 5.25° - 7.5° @ 13" Hg
VC-173 7" Hg 5.3° - 7.8° @ 10" Hg
VC-184 7" Hg 7° - 10.5° @ 11.5" Hg
VC-185 9" Hg 7° - 10° @ 15.5" Hg
VC-187 10.5" Hg 1° - 4° (7° ?)
VC-189 7.0 " Hg 0.5° - 2.5° (11° ?)
VC-208 6" Hg 10° - 12° @ 9" - 11" Hg
VC-239 7" Hg 7° - 10° @ 11.5" Hg
VC-244 9" Hg 7.5° - 9.5° @ 6" - 8" Hg

With 8° of initial, my PN P3690788 distributor provides 30° of centrifugal advance (all in by 2400 RPM) and 44° of total advance.  I bought a VC-208 (11° distributor advance) vacuum advance pod (used on the Feather Duster) to see if I can run a lean fuel mixture that requires 52° of advance.  The HEI system upgrade is to ensure that these lean fuel mixtures reliably ignite to avoid a lean surge.

For more information about adjusting Mopar vacuum advance, see How To Limit and Adjust Chrysler Vacuum Advance Cans.