'78 slant 6 hole in head

You can get a pipe thread tap, and put pipe threads in the hole and then put in a pipe plug. Just makes sure the metal chips don't get inside. Cast iron usually taps dry. Maybe post a pix of the are so we can assess it.

Are there bolt holes on either side, you can make a plate and bolt it on with some exhaust gasket material and copper RTV. You can buy that at better auto stores.