With aluminum I skip the easy outs and just drill through the bolt with progressively larger bits until I can get the threads or whatever is left of the bolt out. I have done too many to remember like that on outboards.
IMO it is better to drop the motor (easy on an XT350) for an easy shot at drilling that bolt out than to pull the head off. Besides if you screw up with aluminum any decent welding shop can fill and retap your screw up.
I am a strong believer in high temp anti-seize for those bolts.