Hmmm... I think I saw that ad. If I'm right, the car came from Covington. I was considering it.
Sorry that you're having issues.
Here are my thoughts...
Pulling it as-is to sell... you're not going to get much for what is, essentially, a "core" engine.
If it's just a head gasket, it's a pain in the azz, but it's not "uncharted" territory. I bought a new/rebuilt 3.4 from Grooms, that blew a head gasket due to a broken head bolt. Stuff happens. Fortunately, they warrantied it.
If you go to the trouble of fixing it, I would keep it and build your 3800, according to your own time frame.
Edit - If you have an air compressor, I would try pressurizing the cylinders with compressed air. If you already have a compression gauge, you can take the spark plug fitting, and screw it into an air hose fitting that will snap into your air hose. (I've done this. It works. It's how I found my blown head gasket.)
Remove the thermostat cap and thermostat, and pressurize all the cylinders, one at a time. If one of them is leaking into the cooling system, you'll get bubbles out of the thermostat housing. Also check your oil, and make sure that none of the coolant is leaking into the engine. (The oil on the dipstick, and elsewhere, will look like a mocha mikshake. If it's already that far along, there's a chance that it's damaged your main and rod bearings.)
With that said, I'm really surprised that you're getting coolant into the intake. That's not usually how it works, but it's not unheard of. Coolant would have to leak into the cylinder, and go backwards, up through the intake valve.
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