Blown Head gasket...Engine gone???

[1980]

Here's the story. 1980 350, Truck was running hot for 3 days, not boiling over. Replaced thermostat and ran fine for 2 days. Third day driving home, about 50mph and normal running temp, Engine, all of a sudden, just almost died, no power and white smoke pouring out exhaust.
Milky oil, don't see oil in rad.
Just a gasket or is the engine toast? Any easy way to tell?

 

[4X4HIGH]

Blown gasket or cracked head. Bottom end should be fine once you do an oil change and fix the top end.