Yes I`ve checked all that stuff. When you start it, it spews coolant out of the radiator filler hole. Something is very wrong. I`d like to sell it as is for $ 1000
So you took your thermostat out and checked it by dropping it in boiling water and seeing if it opens? Have you replaced your filler cap? Checked for air bubbles in the system?
Did you try a compression test? If exhaust gasses are leaking into your coolant system it should show up on a compression test.
Sorry man I don't know what you did or what your skill level is. Just trying to cover all bases and maybe save you some aggravation. If it's running fine except overheating and the only symptoms are water coming out of your radiator filler cap I'd try the compression test. If it fails on one or more cylinders it's probably your head gasket.
If it is the head gasket it's gonna be tough to sell it even at $1k.