Another feederhouse speed question

I will be using a maxflex 1200 platform and JD 618c non-chopping cornhead this fall on a 760. My combine has fixed drive feederhouse that i think can be set at either 420 rpm or 320 rpm by changing step pulleys. Is 420 rpm too fast for the maxflex? Ive ran the cornhead in the yard and does not seem too fast for it, but time will tell in the field depending on yields and speed. I know the rpm reading is for the upper shaft and not the lower so i dont know what speed it translates to.
Any help would be much appreciated.TIA Iroquois Farm

Another feederhouse speed question