How lean is too lean?

You are to lean when the leanest running cylinder starts to detonate. Dual plane intakes are the worst for uneven fuel distribution. Read each cylinder plug with a magnifying glass and stagger jetting will get you close, but a carb motor is all about compromise. Best to be slightly rich then lean. A stiffer power enrichment spring may help your situation.
TPI is not even perfect in today's motors. 02 sensors are only reading an average of the bank so a dirty injector, low fuel pressure, etc can run a cylinder or two lean. The knock sensor then retards timing and sets an engine code.