Manifold Vacuum Advance what am I not getting?

340,
I have been using MVA since the late 70s. I realised the benefits back then. I had Chrys cars until 1995, when I bought a GM car. Chrys did NOT use MVA & missed the bus.....
GM did use MVA until emissions forced the useless PVA on us in 1968. In the 70s, I could not get adj VA units in this country.......& it was before you could get hi torque starters that would crank the engine no matter what the timing was. I was rigging up solenoids on the side of the dist so that engine had Chrysler's [ C ] favourite 12* BTDC initial timing, & once running the sol added more timing. Night & day difference to running.

Your engine.
I am not sure about all C dists, but some I have seen with adj VA units only provide about 22* of VA. In your case, I would get a HEI dist for your engine. You do NOT need an expensive dist, you are only paying for the name. I can buy a Chinese HEI here for $100, comes with polished alum body, all new, well made; I have used heaps of these, never had a problem. The benefit is that the adj VA unit that they come with has about 30* of travel.
I shake my head sometimes....people worried about what what their timing is at 6000 rpm, when they are only there for a second......compared to idle & cruise where the engine spends a LOT of time.

[1] If the engine really is 12:1, then it is going to be tough to use a lot of timing & not get detonation. Nature of the beast.
[2] With a tight c'ter, you will always have a large drop in rpm going into gear with a big cam. Big cams shift tq from the low end to the top end. With that tq gone/lower, the engine struggles with the c'ter load, hence the big rpm drop.
[3] Using the above dist, turn the Allen Key [ AK ] fully CW. Aiming for 40* at idle.
[4] Try 18* init. That means VA adds 22*. If you get det, try 12* + 28*.
[5] To limit VA plunger travel, you have to make stop out of scrap metal; mount the stop under the VA mounting screw nearest the vac pod; measure the length of the plunger movement, it represents about 30*. By proportion, calculate 22* of movement. After mounting the stop, make sure nothing binds. I put a bend in this stop & I can fine tune the reqd movement by flattening out...or increasing the bend.
[6] Centri curve. These dists have too much travel. On the T bar where the rotor mounts, there is a hole inboard on each side. I use a small nut/bolt in these holes to limit the travel of the centri mechanism. Nut goes on top; very accurate positive stop method. You will likely need to file or grind the nut to limit travel. Be careful grinding/filing near the pick up; the magnet will attract ferrous particles which will short out the air gap...& no spark. On the rotor, where the mounting screws are, there is a boss; that sometimes hits the nut. You will need to grind the plastic around screw boss so that the rotor sits square on the t bar without interference.


TUNING.
Ready to fire up & test.
If you get det:
- turn AK 2 turns CCW & check timing. Keep going 2 turns CCW & check timing. When the timing drops, or becomes unsteady, go 3 turns CW.
Done.