Since you are talking 6X-8 heads and a 400 engine these heads cannot be milled enough to achieve 9:1 because of their large chambers. I have been through this exercise before with my block\head combo (same as yours).
You will have to get smaller chamber heads or zero deck the block then start shaving your chosen head to achieve your CR. You will need to verifiy all this with a CR calculator.
If you mill heads or zero deck the block this will change the push rod length and resultant valve train geometry. I ended up using longer push rods because of this. If you mill the head you must mill an equal amount from the intake side of the head otherwise the ports may not match up. Your machine shop can tell you more about this.
If you change to a cam with a different base circle diameter this will affect push rod length.
See my photo gallery for a picture of stock 400 piston with chamfer. This chamfer lowers CR. Same is true with a valve job, it lowers CR. bt
Originally Posted by 77pontolds
I should do to the 6X heads to have aC.R around 9. to1.