'71 318 compression...

7 to 10 cc will make a difference in compression, you might want to keep the small chamber heads on. With 2.76 or 3.23 gears you wont be reving the motor all that much. There is a formula on the Kieth Black site that will let you plug in the numbers and get some kind of idea about the effect. How far down in the hole do your pistons sit at TDC?