There is nothing wrong with using copper spray on your headgasket/motor, alot of people swear by this(helps seal the HG) I have been threw this already, but to be sure check you cam and crank timing like I said, this will tell you alot as to what is going on. if you did bend valves the screeching could very well be the valves having trouble opening and closing if they are bent. I highly doubt that copper spray did ANY damage to the motor, it sound like it was timing wrong(happens ALOT, with newer motor builders ) and someone didnt double check their work. after timing the motor, the 1st thing you should do is rotate the crank a few times 360deg's and make sure that it spins trouble free. If it becomes very hard STOP!! there is a really good chance you are hitting valves, and then you have to retime the motor. Not turn it over with the key... which like stated above would bend you valves in a heart beat LMK if I can help....