Milling down SM 68cc CNC heads?

I got a set of SM 68cc CNC heads on the way from the black friday sale. I really wanted the 65cc heads but they were out of stock and you couldn't order them for the sale. Using a compression calculator from Summit, I am showing a CR of about 9.16 to 1 with the 68cc head. I really wanted to be closer to 10 to 1. If I can get the head to 61cc, it would put me just about where I want to be. Using the "junkyard jewel" build up as rough reference, when they milled the heads .050, they picked up about 6-7cc.

1. Anyone with these heads mill them down some to pick up some more compression and what amount would you recommend?
2. Am I correct that the intake manifold will now be needed to be milled down as well to fit correctly?
3. Any issues that may arise with this idea?

Thanks.