I would have expected 11-12 in of vacuum without advancing the ignition timing. Where did your idle RPM's end up with the change in timing? If at 850-900 now, then the vacuum increase is just due to that IMHO. If still at 800, then good.
For reference: My son's 340 is almost identical, with the same heads and intake and 10.0 SCR: 268 Crane cam and 112 LSA and slower ramps so the overlap is less, which helps the vacuum, so it is a bit different there. It is at 14" at 800 RPM with 15 initial. Cam timing is at 107 ICL. We had a lower vacuum initially (11"), but fixed the PCV and also fixed the secondary stop screw setting and that brought the idle vacuum up to the 14" range.
What PCV do you have? Do you have headers?
Also, are you racing this on the strip or just cruising? I ask because with your SCR and the cam and other parts, you're gonna have plenty of low and mid RPM torque, and I'd not be pushing the initial very hard with that SCR, unless I was trying to wring out just a bit more time on the strip. But, I suspect with your engine, getting the tires to hook up with all the torque that you already should have (and that the dyno torque numbers says you have) is a much bigger deal, and putting earlier timing into it just makes that harder to manage.
So understanding what you are wanting to achieve here is important.