Do you use a timing light ?

[scooder]

Jim, you got 10-12 initial and 24 max already built, as Dale said by all meens try bumping you dizzy a couple of degrees see what happens. Even though you got your Canadian heads (slightly higher compression) it's quite an open chamber, plus with tight quench cooling the charge it can in theory stand a bit more advance before ping or knock sets in.
Your 10-12 initial and 24 max is about where I would start with your engine combination. Have you tried allowing the advance to come in earlier? Backing off the vacuum brake would allow this.
Martin.