I suspect you’re a little too much LOP which is causing the stumbling. Id recommend starting LOP to try a setting that will get you 65% so you can’t hurt anything and then play with it from there. You just need the FF for your engine. As long as you’re on the lean side of peak, FF is the only determinant of power. So your engine, 65% is 210hp x .65% = 136.5. Divided by 13.7 gives you a ff of ~10gph. Try that ff at say 27/2300 or so maybe 28/2300 if it will run smoothly. set the mp/rpm, then big mix pull to 10, then reset the others back to 27/2300 or 28/2300 as required.
Then slowly richen until peak to see how far lean of peak you were (and then set back to 10gph). At 65%, you can shoot for closer to peak, maybe 20-40degrees. If it’s too lean, pull throttle back just a little as that’s what is controlling the air into the cylinders.
you should see much cooler cylinders.