While working in a Buick dealership I had occasion to head gasket dozens of 350 Buick engines due to premature deterioration of the fiber head gaskets Buick used.
By the time they were brought in for head gaskets they're had been pouring coolant into the exhaust for awhile. After completing the repairs, they belched steam out of the exhaust for 20-30 minutes. It takes awhile to turn all that coolant to steam. Knowing this I would say by the time you run it for 15 minutes you should see a huge reduction in the amount of steam. If it hasn't abated by 15 minutes of run time, you might be looking at some other problem.
Hope it all works out for you..........
