There is an old hot rod magazine article available at
http://www.selectric.org/55chevy/soup.html It should answer your questions. Interestingly when the 261 that is the subject of the article was bored out to 278 cid the article states the compression was raised to 8.5:1. I'm thinking it must have been the pistons they used.
If I remember correctly they did use an 848 head as part of the modifications and there is some discussion of that.
Lots of good information but maybe outdated a bit by newer equipment/methods but maybe not.
Jerry
