Looks like she is running a bit on the lean side. But it's really confusing.
She runs just fine, starts easy, makes a lot of power, no knocking under load etc. - not a single sign of a lean running engine, at least as far as I know.
I have a ton of advance though - 36 deg. (at 3600 rpm) mechanical and 20 deg. at 15 hg/inch at the vacuum (manifold). But actually if you drive, the Engine loves it, and I think it would be happy to take even more. A guy that actually was racing 235s/261s back in the 50s and 60s told me an 848 head wants a lot of timing , and that's exactly what I see. The more I throw at her, the better she runs - but that white plugs are concerning me.
This is a mild build 261, 848 head, 3/4 cam (Bulldog), overbore, single Clifford intake, headers, flipped pistons, Fish carb.