For best fuel economy
Have you considered that the Cruise timing, depending on rpm, will want to be mid 50s., say 56* @2600 rpm ( say 61mph with 3.23s and a modest stall TorqueFlite)
To get that much timing, your V-can may need to be modified. the factory Mopar one can be modified to in the range of 22>24.
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In this discussion, I will be talking only about fuel economy, with an automatic transmission..
Your Secondary curve points to 16* ending at 3200, but does not state when it begins. If I assume it begins at ~1000rpm; thus the curve is 16* per 2200rpm, therefore .727* per 100rpm,
Therefore, by the math, at 2600, this would add
11.6* to the 20*@ idle= 31.6 to which your VA can add up to 16 at cruise
for a total of 31.6 + 16 =~48 ; That's only getting close to 56, but 8* more would put it darn close.
The problem is this;
cruising at 30mph in Second gear at 1750 rpm with, 20+5.5+16=41.5 degrees which may be/probably is, not enough. You opted to take the Va down to 10*, so you should be getting 35.5, which is not nearly enough.
IMHO, what you should be considering;
is taking Idle-timing out,
keeping the all-in Power-timing near the same,
Plus modding the Vcan to 22/24*;
and in this exercise, that 280 Purple should IMO, not be saddled with anything less than a 2800 and 3.91s.
Ok so, with 3.91s now, 30 mph in Second is now 2100, and the Cruise timing will be
14* idle
plus mechanical timing will be .846* per 100, so is 9.6 .
plus up to say 22 in the can Which now totals 45.6. But when you stomp it, the VA comes out, leaving you with ~24total, but the Convertor is jumping up to 2800 where the Total is 14* plus 15 =29, and in the blink of an eye will be 36*@ 3200. If it detonates at 3200, delay it. Maybe at 3400 it will be fine IDK.
Now; 65mph with 3.91s will be around 3200 depending on slip. At this rpm, your timing could be;
14 Idle +19 mechanical +say22VA= 55* Cruise-Timing so now, yur really in the ball-park.
Plus the low speed off-idle circuit will love you for it, with your Mixture screws back where they belong.
As to idle fueling; What you need to know, is that at Idle, your engine is getting whatever fuel it needs from the combination of Transfer slot fuel plus Mixture screws.
The mixture screws cannot provide ALL the idle fuel, but with the timing cranked up, almost. You can therefore set the idle timing to whatever you want, until the mixture screws run out.
The trouble is, now the Transfers are as good as shut off, and the engine is pulling in nothing but air from the Idle-wells, and so, when you tip the throttle in, those transfers take time to wake up again. This usually causes a Tip-In Hesitatation, maybe even a stall if yur not careful.
But it doesn't stop there. As soon as the low-speed circuit wakes up, it goes very rich because the mixture screws are cranked wide open. So then, about 100% of the time, that you are cruising on the transfers, the mixture will be overly-rich, and you blame it on the cam.
Well, truthfully, that cam does suck gas alrighty, especially so on a low pressure engine, which makes it all the more important to be able to run the mixture screws lean! not rich.
The trick is to run the mixture screws a hair lean, and have the transfers supply the rest of the Idle-fuel, then control the Idle-speed with idle-timing. It the exhaust starts to burn your eyes, you add bypass air.
Now, I can tell you that when I was running the 292/292/108 cam It would idle down to 550rpm @5* advance, and still take throttle! So, don't let anybody bully you into telling you that you need to have a certain amount of Idle timing.
Here is the shortcut.
Run your rpm up to 2000 and fix it there.
Now twiddle the mixture screws to best lean running.
Next advance the chit out of the timing, a little at a time until additional timing does not produce any more rpm.
Reduce the idle speed back to 2000.
Twiddle the mixture screws again to see if you can go leaner.
Now, read your timing.
This is the exact setting that your engine wants, to cruise at no-load, at 2000 rpm. You cannot make it any better, now the rest of the bad fuel economy is on the cam, lol.
Idle it back to ~750. and take out 3 degrees for anticipated load at 2000. again read the timing.
Whatever number you get at idle, is, at this point, fall out.
Now, Without changing the timing, and with the VA defeated; rev the engine up to, whatever rpm gets you the most amount of timing, and make a note of the amount and rpm it takes to get there. If this number is less than 34* at more than 3400rpm, You may want to change that. The trick is to create an advance curve that runs from the previously established cruize-timing at 2000, and to approximately 34*@3400.
What it then arrives at, at idle, is again fall-out.
The only way I know of to change that idle-timing, is with a two-stage timing curve. and that is a lot of trial and error.
But, honestly, with a 2800 convertor, Your engine doesn't hardly care about idle timing. The first time it cares is at stall.
Thus, really, you have only three data points to hit;
Cruize-timing,
Stall-timing, and
Power-timing.
Let idle-timing be what it will be.
So keep after it, I know yur having fun, and you're closing in on the home stretch.
Happy HotRodding
PS
Now if you have a manual transmission, it's a whole different ballgame.