Cracked Head?

[Spline64]

Hey all,

Have a Chevy 350 motor in my GMC C2500 '73 3/4 ton pickup. Original was a 454 but someone replaced that before I purchased it. I would post in the 73+ section but don't have permission to post.
Was smoking so bad had to change the valve seals without removing the heads which did fix the smoking problem but during that process noticed that when air pressure was applied, the left head was pushing back coolant to the radiator in two of the cylinders. Figured I had a bad head gasket so I waited till my vacation to pull it and replace.
Well now it's vacation and I'm not sure that what I think I found is even possible. I see what appears to be a hairline fracture on the bottom of the head that travels up to the exhaust valve. The intake valve is also shiny compared to the other intake valves. The spark plug from this cylinder is normal with no buildup (dry).
I didn't perform a compression test as 100 psi air pressure caused it to leak back so knew something was wrong. the engine is gutless as heck. Never hits passing gear except from first gear and basically half throttle is the most you will ever get for acceleration. The gasket looked fine so was wondering if any of you have seen a leak through the actual head that could cause this type of behavior?
I have attached a photo but hard to see the actual crack.

thanks guys,
Pat