All Remember, temp gauges dont read correctly when the sensor isnt surounded by water..3/4 temp on the gauge with no water? the smoke was prob the old oil being fried of the engine block while the now very thin and burning engine oil in the sump is tearing up the bearing and camshaft.
one an engine has this sort over heat, youll prob find she is now burning oil and the gasket will now fail....
Stop leak is a last resort thing on the side of the road in the middle of woop woop. If it worked all the time the rad specialist wouldnt be around.
The rad was prob fine with no pressure build up due to the leak but after you put in the Stop Leak she sealed the leak causing the pressure to build up, blowing out the next weakest part of the cooling system. If you were watching the temp gauge you would have seen it drop a little as the water disapeared around the sensor then start to rise...Too late.
To replace a head gasket, A true work shop wouldnt just replace the gasket is its got hot..The head would be warped and / or cracked. To give you a price is like reading my crystal ball.