i have mill tap them and put guild plates in the chevy and olds and ford heads it is not cheap so if you love them heads and do not want a better head them fix them. but they do not need guilde plates. if mild engine .to do this is much more work and money and you have to drill the pushrod guild holes out of the heads and will need harden push rods to they make a 7/16 bottom and 3/8 that threads in and is made to fix a stock head with out guild plates