Also, that dooflinkie spacer that you should have betwixt the "rot pot" and the manifold to keep the gas from overheating(vaporlock and all that)should be secure and tight, some guys use gaskets on both sides, as I have, the manual says its not needed, and we can all fight about that until the next war, but I noticed a HUGE difference in fuel milage after sealing it with bathtub sealer, some paint to make it pretty, two hail Marys and some salt thrown over my back for good measure.As far as piston slap goes, after a minute or 2, it should be gone. I have a 409 that has horrendous slap when cold,as these motors are known for and as scary as it sounds, theres no problems,Chevy dealt with this issue late '62, but my '65 "nine" still sounds like a box of rocks when cold.
