Mine was weeping ever so slowly when under pressure for at least 10 years. Lots of rust on that side. I didn’t realize it until the head came off since it’s kinda hidden.
Your freeze plug has been in your cylinder head for 35+ years. It’s time tested -though maybe weeping a tiny bit when under pressure.
It’s seems that the chances of it blowing are extremely slim. Maybe none. Nobody on Mud has ever reported a blown cylinder head freeze plug that I’ve read.
Now that you’ve identified that potential problem spot, maybe just clean up that area and keep an eye on it until the next time the head comes off.