Strange source of an oil leak????

[10drum]

Greeting,

I have a 69 C-10 with a 307, just under 50K, yes it's actual milage. I've been trying to run down an oil leak. It was coming from the back of the driver side head. I had changed the valve cover gasket, that wasn't is it. I changed the intake manifold gasket and was able to get back there and clean everything real good, and the leak seems to be coming from the head gasket???, so I pulled the head and found this plug, See Photo. It's tucked up under the head. It looks like it is a port to the oil psi take off. I'm not sure but I think that may be the source. I'm not having any luck getting it out, it's a 1/4" drive but I need a good hard drive because I've already broke my 1/4" break over and a 3/8 to 1/4 reducer. I'm letting it soak tonight and will try heat tomorrow. Any suggestions? Thanks for all replies. Phil