91 Octane in a 340?

Inlet air temp, combustion efficiency, timing, and carb tune all can effect detonation. If you run 91 you may just need to back off the timing some and/or work on the carb tuning. If it get hot where you live then run 94 durring the hotter months as a margin of safety. Fresh air ducts to the air cleaner help a lot too so you're not ingesting hot engine bay air.