Don't try any stop-leak bottled mojo. They're generally only good at clogging up your heater core.
First thing to check is the thermostat flange/gasket. If it's your lucky day, that's the source, and a new gasket might get 'er done. Also, the flange on the gooseneck is thin and prone to bending / warping. There's available a "reinforcement" flange that goes behind the flange, along with longer bolts, that keeps the mating surface flat. I just don't remember who sells it, whether it's an org vendor or a vendor on one of the other forums. Bookmark link is on my computer at home.