Erratic Timing

Mattax said
That's about right, but clearer to describe it as timing at high speed cruise (60 - 65 mph).
Lets say the engine is turning around 2800 rpm at 60 mph, and you observe the vacuum is around 17"Hg.
Then the mechanical timing at 2800 plus the vacuum advance should equal 50 * BTC.
example:
The '68 318 graphed above shows the distributor with 5* initial. At 2800 rpm it could have a timing at 24*.
At 17" Hg it would have no more than 27* more, probably closer to 26*. 24 + 26 = 50* :)

OK now that makes sense. And of course that would explain why people drive their cars to get the vacuum advance dialed in, and how adjusting the amount of vacuum needed to begin the advance would also affect how much advance you actually get at a certain speed.

Mattax said
LOL. OK. Do you think its a basically stock rebuild?

Well he SAID high compression and mild cam, but I'm getting 18 hg at 1100, so it must be pretty mild. Fine by me, this is purely a streeter. He threw on edelbrock intake and headers though, and got rid of the egr system.

Mattax said
On a early smog engine, you're actually not that far off. Plymouth spec'd the '68 & '69 smogged manual transmission cars with 5 *ATD initial.

That said, those early smog setups are a PIA and since you're not doing a points resto, there's no reason to try to that.
Bet if you give it some more initial, you can then turn the idle speed screw (aka idle stop screw) down and get a slower idle speed.
Then fiddle with the idle mix screws. Turn them in until the speed drops a little, then back out roughly 1/4 turn richer.

Do this in a small steps. In another words repeat the process a couple times. As the engine warms up, you should be able to get idle speed down to 600 - 650 rpm.

That's next on the to-do list. Hopefully the timing will actually make the difference this time.