Cruise AFR is primarily determined by combustion chamber and atomisation. I understand ignition can have a lot to do with it, but when dealing with the ever-familiar Honda I see some engines starting to stumble in the 15s, others 17-18 AFR - one was an LS the guy claimed 46 mpg, I was surprised. Anyway. Give the setup what it likes.
As far as timing, the "bigger bore likes more timing" adage seems slightly amplified, compared to WOT, be a degree or three. Also, CR and presence of EGR make a difference - as does AFR, lean might want more timing. Frankly I don't hear many/any part throttle cruise timing pings. I think its the sort of thing you want to steadystate on the dyno and tune for best cruise at torque, but knowing jack shit about your motor I want to say set cruise timing to 38-42 deg and forget it until you buy time on a loading dyno.