slant 6 head shave

I just bought a 68 two door dart with a slant 6. My other dart is a 440 swinger which was originally a 340 car. I have no prior experience with the 6.

Here is my plan. I do not want to pull the engine. I want to cut just the head. So I need to know how much to cut off of itto get me to at least 9.5:1 but no higher than 10:5:1.