I have not used them and in fact I have 906 iron heads on my 63 but from what I have heard they are a great deal for the price. They are just about the same as the Eddy heads and should work good on a motor up to 600 hp with some work. They are definetly better then 40 year old factory iron heads. I would not hesitate to use them on a nice bigblock build for a street car as you cant beat the price. Ron