I just went through this on my 67 383. I bought KB pistons and on paper I was to be at 9.0:1 compression. Measured came in at 8.2:1 and I cried.

I fortunately was able to recover by milling the 915 heads 0.040 inches and that got me to around 8.9:1 (from what I recall - could be a tad less). But that was with a closed chamber head - the open 906 head puts you in a much worse position.