[beckerb]

I worked for Ford putting myself through school back in the mid 60's and early 70's (7 years bending wrench and all the Ford schools that where available at the time). Ford had a ton of problems with the 428 cobrajet related to value guide problems. Back then we did not replace the heads, instead we re-worked the value guidels. I mean hundreds of them - what a pain!!!

If you think you have a guide problem it is easy to check. Remove the value cover and with a larger flat blade screw driver see if you can rock the value in it's seat back and forth. Position the screw driver shaft on the value spring retainer and the blade on the head. If you get movement that you can really see, you have a value guide problem.
Perhaps I am old school but if you have piston slap due to either piston, ring or bore problems normally you will experience a loss of compression during a compression check. Although it can be somewhat small, it will be there. As a rule, you will also experience oil fouling on the plug(s) where the piston is loose in the bore.

Just an ole timers view .....