When these heads were assembled, did all the springs get checked \properly? seat pressures height ect.... how many times had the engine rev's gone above 6400rpm since these springs were installed? When the 2 springs broke were then in the same spot or location? did you check for any coil bind after the cam was degreed in and the top end was assembled? are you running a stud girdle for the top end?

The only thing i keep thinking about is when you get into the 6500 and up rpm range that possibly something is causing high stress on the spring and failure. maybe the pushrods are to long and causing the high rpm bind and snap the spring fails. was there any marks on the inside coils of the broken spring like it had been making contact with the seat?