Engine started and went immediately to 5000 rpms, work completed was thermostat, water pump gasket, intake/exhaust gasket, freeze plug.

Forgot to connect the exhaust but wondering what next to look at?

 

Just a guess but I'd suspect an intake manifold to head leak

 

That would be you intake manifold not seated properly. It is tricky to get on right, especially if the little guide pins don;t go very far into the manifold. It is really easy to feel like itt's lined up correctly. The boss on the intake should be a little bit below the top of the cylinder head. Been there and done that plenty of times. You get good at find a way to get those lower manifold bolts loosened/tighteneted.