After a new distributor with an adjustable mechanical advance, a new timing chain along with advancing the cam 2°, a set of correct heat range plugs and some carb adjustments I've achieved my goal of 10 MPG well actually 9.89 MPG. Close enough! Still have some very, very light spark knock under a load even with 89 octane fuel, but it isn't anything down shifting to a lower gear won't cure. I'm happy with it.

Attaboy - for that last .11, air the tires up, and imagine an egg between the gas pedal & your foot.
Might even ponder some aero, like smaller mirrors or an air dam, if it suits you.
On my Dually, I lift early approaching anything I might have to pick up the throttle from afterwards; makes a difference.
Coasting 5500 or so pounds instead of charging into the future, so to speak, helps.

did you check your afr? i would look at the carb next, get yourself a big spreadbore so you have the power when you put your foot into it but only sip the fuel at part throttle on the primaries. are you running vacuum advance? that helps alot at part throttle.

Oh yeah, I'm sure I could eek out a few more tenths if I really tried.

Not changing carbs, don't have an O2 sensor and I'm not running any vacuum advance. The mechanical is set at 12° BTDC at idle and 36° total all in by 1700 RPM. I know a vacuum advance might help some along with delaying the full advance but I'm not messing with it for now, it runs way too good. I started at 7.3 MPG I think, so I'm pretty happy with it.

I have found some low hanging fruit on old carb/distributor engines with a little vacuum advance, not a ton but a little bit goes a long ways.

there is a reason the factory ran vacuum advance, and no it is not for emissions, my 51 Plymouth has one.

Run one, it will help.

Sometimes yes, other times no, depends on the combination but might be worth a try. If it starts surging or misfiring you have added too much

Good job on the ‘74

There in your driveway you have two “Bookends” of the crisis years of oil production
from 1974 to 2019

The vacuum advance is adjustable on this distributor so I may hook it up to the ported vacuum source. The engine doesn't make a ton of vacuum, even though the cam is mild it has a 108° LSA, 60° of overlap IIRC.

Guitar there is likely improvement to be had in your mileage using a vacuum advance. The more part throttle cruising you do on the highway, the bigger the difference in mileage will be. At some point I would revisit it, but can I certainly understand enjoying your ride as is for now.

X3 on vacuum. Also do you have EGR passages? Lots of EGR and advance at cruise can help.

No EGR and the truck spends very little time cruising on the highway. The mileage I recorded was with 28 miles on the highway at 2K RPM out of 125 miles of driving. And the A/C was on.

EGR does not help MPG, it is strictly there for emissions. It actually lowers MPG and is the reason manufacturers are going away from it. Air is 21% oxygen and you need that oxygen to burn fuel and you need to burn fuel to make power and move your car, that fuel needs to burn extremely fast and the less oxygen in the air/fuel mix because of exhaust dilution the slower it will burn and more likely it is to continue to burn after the exhaust valve is open and it can no longer push on the piston so it is wasted and statistically less of the fuel molecules will actually burn in the cylinder and it will reduce efficiency. At 20% EGR you have reduced the oxygen available to burn any particular molecule by 20% and there is only now 16% ish oxygen in the combustion chamber so our fuel molecule is less likely to find a partner and if he does find one it is more likely to happen after it has a chance to add pressure to the cylinder. You are also going to increase carbon deposits in the intake trac including the throttle body or carb, intake manifold, intake valve, piston top and combustion chamber. You are also pumping more molecules through the engine further reducing efficency, that is why the new hemi shuts off 4 cylinders and not just the fuel to them but the air that would otherwise continue to flow.

Yes EGR slows the burn, that's why you add timing. It can also help with pinging as it lowers peak pressure.

Add even more timing so you can do even more work against the rising crank.... got it.

>> The mechanical is set at 12° BTDC at idle and 36° total all in by 1700 RPM.>>

That is already a good amount of advance.

If there is a slight audible pinging when climbing a highway grade the timing “might” need a bit of tweaking.

I have run with an Actron ODB-I scantool hooked up on a 1995 360V8 and seen as much as 47 degrees of ignition advance at 1600 rpm on a level Interstate,
but the 1995 has Magnum high swirl heads, EGR and SMPI. I do not remember the manifold vacuum when that happened but would guess 10 to 12 inches of Mercury.

It does have magnum heads on it I've tried running more timing but it doesn't like it. 35-36° seems to suit it fine.

What I used
http://www.mopar1.us/engine2.html

35 - 36 seems to be the max mechanical that a magnum head likes but they do like more at light loads and that is where the vacuum advance should come on. They seem to like about 17 - 18 at idle and mechanical should come on pretty slow, this is one of the reasons the factory magnums got such bad MPG, way too low advance at lower engine speeds and idle... that it jumps around a ton at idle, not steady at all and I'm guessing it was done to make it sound a little more powerful because when you put a carb and dizzy on a stock engine they idle a lot smoother than the factory EFI setup.