Manifold Vacuum Advance what am I not getting?

Just so you know,
I built a 367 @11.3 and 8.8Dcr pumping over 180 psi; and it idled at 5*Advance. This was with the Mopar 292/292/108 cam; and with OOTB Edelbrock aluminum heads. I ran it on Ported vacuum. But I modded the can by filing the stops until the diaphragm bottomed inside the can, and got 22 degrees. Using the same trick, Trailbeast got 24* . Any Mopar VA can be so modded. Not every combo will like 22 degrees. Well, any VA in my collection.
This engine was for a Manual-trans car, and at 5*, it had just enough power at 550 rpm, to pull itself around the parking lot with 3.55s.
It's "normal" timing was 28* at 2800, going to 34* at 3400 This gave me 12>14 at idle. I ran this, exclusively, on 87E10.
Initially,. with a regular 4-speed, it liked 56* at 65mph, but the more timing I ran, the leaner I was able to cruise at.
Just saying, don't shoot the messenger.

BTW,
The 292 cam was real hard on gas cruising 65=2900. I slowed down to 55=2470; but it was not much better. I pulled it out and sold it to a racer friend.
After running two other cams in this engine, I figured out why it was hard on gas.
1) In at 104, it only had 102* of power extraction, so there was a ton of energy still in the exhaust, which could have gone into the flyeheel, and
2) this cam had 76 degrees of overlap, and actually didn't much like running at 2470. It liked 2900 just fine, but now the rpm was burning up the gas.
If you know about overlap, you know that the primary purpose of running headers is to pull on the intake in the overlap cycle, to get the Fuel charge moving towards the chamber, before the piston begins it's downward march with the Exhaust-Valve closed. Well at low rpm with so much overlap, the Headers were doing a bang-up job, pulling fuel charge straight across the piston and out into the pipes. Fat lot of good that did for me, Like I said, I pulled that cam, towards the close of it's first summer.
Thinking I had learned a good lesson, my next cam went the other way; 270/276/110
In at 109, this cam had 111* of power extraction. The Dcr went up to over 9.2, the pressure to over 195psi, and this thing ran like the proverbial scalded cat.
The overlap came down to 53*(from 76), and End of reversion moved down below cruising speed.
I hung a Mopar A833overdrive on the back, and a GV overdrive behind that, and geared it for 65=1600/85=2100 And fuel mileage jumped to over 30mpgs. 'Course it required a new timing curve, which for me is not that big a deal, cuz I have a dash-mounted, electronic, dial-back timing module with a range of 15 degrees.
This was my most awesome combo. I split the overdrive gears, with the GVod, and got SEVEN useable speeds.
BTW-2
At idle, the engine only needs enough timing so it doesn't stall, and doesn't bang the trans when you put it into gear. The first time the engine really cares about timing is from Stall-rpm to ~3600, at WOT. Everything else is optional. and there's no perfect numbers until you go with a computer.
The more Idle-timing you try to run, the greater will be the bang/rpm drop going into gear, and the further away will go the Transfer slot Sync, which usually creates a Tip-in Sag/hesitation, which when severe, ends in a bog or stall..

Long post I know, but that's the way I roll.
The point is, 12.5 Scr with iron J-heads, is probably not gonna fly, no matter what timing you throw at it. I see a compression ratio downgrade in your future