Bench is ok if you keep it cool. Use a shop vac on "exhaust" to provide a direct cooling source to the head. NOT through the prop. Spinners and/or back plates do little to nothing for directing air over and through a cylinder. Only actual ducting will handle that.

No need to do more bench running than a tank of gas through the engine. Just enough to set the needles close to what will be needed. The rest of the break in will be best performed in the air.