Vacuum advance tuning to maximize cruise MPG?

BBM; yes Im hooked to the spark port.

md;
thanks for the pn.Unbelievably, I have a complete 360 sitting here from a 77 van. I will cross reference and see if maybe I have had one on the yard here for 10 years already.
-Ok, Thanks to BBM for pointing out that you are hooked to full manifold vacuum. I missed that. Partly because of the way Ive been reading your posts. Are you still running it that way?
-The reason I ask is because it may limit your total possible advance.Let me explain. Lets take as givens; theres 18*@14" in the can, initial is 24*, and power is 34*, and it idles at 11inches.
So lets say at 11 inches the can pulls 14* OF THOSE 24 INITIAL measured on the timing lite. That means 24 less 14 = 10 actual initial. Since we know 34 is the max also measured the same way; then there is 10* in the dist.cam. So then when the vacuum drops out at WOT, theres only 10 + 10=20* of power timing. And the max cruise timing would be 38*( 10+10+18=38 ). I think Im reading your post wrong, somehow.
-Im wondering if you didnt actually set the timing with the can out, at 24* and then hooked the can back up and are then idling at 24 +14@ 11"Ima guessing at this number)=38*. Power would still be 34* and cruise could be as high as 46@ 2600, assuming 6* centrifugal of the 10* comes in that early.So as to the 56*, I am again bewildered.My math works out to (24+10 mechanical)+(18 max in the can)=52.

-Consider how it works when youre bombing around.
-It kinda works ok, except under light throttle, low rpm, the advance is always going the wrong way.The advance provided by the can is falling with falling manifold vacuum just when it needs to be rising with rising vacuum that would be provided by the spark port.
-You have to understand how the sparkport vacuum works compared to manifold vacuum.If you installed 2 V-gauges and put them in the cab,you will quickly see what Im getting at.
-Try to imagine some driving situations, and you will soon see what Im talking about.These situations must involve part throttle accelerations going to light cruising, and the reverse thereof.Then when you stand on it a little harder, say at 10"vac. there is way too much timing in it. Only when you go to WOT does the V-can finally go to near zero advance.
So it works at idle and WOT and maybe a few places in between, but a lot of the time, say between 4" and 10" vacuum the engine will be seeing too much advance.
-Your combo might be Ok with it (9.5c/r iron open chambers), But I know my Eddies at 11.2c/r, did not like it.
-Another thing I did was map out my ignition curve and can curve.This so I knew exactly how much advance I had at any rpm and vacuum at the sparkport.(with a vacuum gauge teed into the sparkport line).I strongly urge you to do the same.Then you will quickly see what i may not be explaining that well. Then you may decide to put the vacuum source back on the sparkport.

-Youve got some great ideas, and are pretty focused about it, even to the extent of purchasing the dial back timing device.Your way will lead to better economy,thats for sure, but using the sparkport will get you the exact same cruise timing, and not be risking your bearings.Having said that, I can see you are no newbe at this, so if it works for you,( that is; detonation free ), then I will have learned something along side of you, and will return the thanks.
Alright, enough for now. My head hurts.
Did I read you were going on a roadtrip? Well then; Have a nice trip.