Vacuum advance

beware the dreaded idle tip-in hesitation that takes extra pump-shot to overcome, and maybe you run out of shot 1/2 second later.....
I gotta repeat;
with an automatic, who cares about the timing below stall..... even 10 degrees short will not be missed.
But if you hit stall at WOT, with even 1* too much, over time, you are probably gonna detonate the sparkplugs to pieces. And those ceramic bits are about three times as hard as steel..... 2200rpm is one revolution every .0273 seconds, which on a V8 is 4 firings. When you hear the detonation, IF you hear it, it might take you .5 second to lift off the throttle. In that time, your engine has fired 73 times................. where are the bits, and what are they doing? Jus saying.....

At 2200 and cruising, your engine might want 56* of timing, for best fuel-economy. If you run 12 mechanical at idle, and an additional 12 in the can, on manifold vacuum, what happens at 2200 and cruising?
I'll tell you;
Your max Power-Timing with a lo-compression 318 might be 36* at 3400rpm. To get to 36 from 12, is 24 degrees. And if your curve begins at 1000, then you have 2400 degrees to get there. This maths out to 1.0 degree per 100 rpm.
Therefore, at 2200 the mechanical timing will be 12initial+12 mechanical =24. And in this case, the Vcan is bringing 12* to the table so 24+12=36 at 2200 and cruising.
But your engine might want 56*! so you are 20* short, and no wonder the thing is a pig on gas.
What I mean is, the window of best timing in a running engine is about 25 to 28 degrees ATDC. If you fire it 20degrees late, the piston is at 45 or more degrees ATDC. So now, the already low-density A/F charge, which was compressed to the max at TDC, is rapidly losing pressure as it speeds down to the bottom, even tho it is on fire!. So the peak pressure potential is never reached. Therefore, to make the minimum horsepower to cruise on, you have to open the throttle further.................. to burn more gas, to compensate for that portion which is still burning in the exhaust system!
How lame is that!
Now
Lets put 22* in the Vcan by modifying the stops. And our target is 56*, so we need Idle plus mechanical = 56 less 22= 34@2200 and cruising
At idle, using manifold vacuum, all 22 will be in. And all 22 will stay in until manifold vacuum drops below the set-point of the springing.
If you manage to mod your D-cam, to 24*, then at 1* per 100, at 2200 will you will get 12* mechanical, plus the 22 in the can = 34, but you need 56 so you gotta set the Idle timing to 56 less 34=22.
But, at idle, with the timing set to 22 and the V-can on manifold vacuum, that would be 22+22=44* of idle-timing. So............. Ima thinking that's nuts.
So instead, you hook your V-can to ported vacuum and it doesn't start coming in until say 1600 at light throttle, but by 2200 light-throttle, as in cruising; it's all in. That's how you get to 56*@2200 for cruising.

I'm not saying your particular engine wants 56*@2200. So you might have to make some changes. But the game-plan is the same, namely, when striving for fuel-economy, most carburated engines I have worked on are sadly lacking in cruise-timing.
But even more sad is that, at Part-Throttle, the timing is almost as bad.
You think the way to fuel-economy is a 2.2 liter engine, Direct-Injected, IVVT, and computer controlled timing? Your car takes a specific amount of horsepower to cruise at a particular speed. in the neighborhood of 35 to 40. The car does not care if it has one cylinder or 16 cylinders, it only cares that it has the exact amount of power to go the speed you want to go, at the rpm you want to go at. And it's NET power, so the more cylinders you have, the more of the crank power goes to friction. But that is a minor deal. The biggest deal is the rpm. and the next deal is ignition-timing. The slower you run it, the harder it is for that ancient distributor to give it the timing it needs.
Since you don't have a computer controlling the timing, it is the tuners job to try and hit as many of the bases as possible, as often as possible. Even the factory sacrificed cruise-timing..... because they had to. If you give your ancient 318 (or other SBM) the cruise-timing it needs, it will, in steady-state running, rival the modern computer-controlled car. It has to; it takes a certain amount of power to cruise 65mph, which takes a specific amount of fuel, and EFI is NOT the be-all/end-all that the hype says it is. Yeah sure, injectors are a lil better atomizers. So they will likely always be a lil better at cruise-economy........... but;
have you ever noticed that from the early 70s to today, economy has only jumped the a small amount that it has? There is still no 100 MPG EFI system out there; even with engine size having been halved. If you look at the average economy of any similarly weight and frontal-area to our A-body cars, what do you see? I see 2.4liter cars doing ~25mpg on the hiway, I mean besides the heavy hitter cars. How many 80s Fifth Avenue cars, weighing nearly 4000 pounds, running in-the-basement-Scr 5.2liter 2valve push-rod engines on carbs , didn't get that or nearly so. With 3-gear loc-up transmissions no less!
The difference is, with computer-controlled ignition systems, those EFI cars can run the timing on the ragged edge of detonation. While us dinosaur-guys have to work with what we got, and most of the time, cruise-timing is at the bottom of the priority list.
If your V-can is of modest capacity, yeah, you can run it on manifold vacuum; but with the modest capacity, you can never hope to achieve impressive cruise fuel-economy, NOR will the PartThrottle timing ever be even close, so you will ALWAYS be driving deeper into the throttle than you should/could be, so economy suffers.
If you simultaneously have a low-pressure engine, well it's no wonder those engines get a bad reputation.
And heaven help you if you have a manual trans; you loose the Torque-Multiplication in the TC, and you loose the stall, and worst of all, the tires are married to the engine, at low rpm almost all the time.