boring my 318

Your compression is an overall figure based upon the volume of stroke compared to the volume above TDC, which includes volume of cylinder above TDC and the cc's of your combustion chamber. To increase compression ratio, you can either use taller pistons, reduce combustion chamber volume ( with different heads or milling ) or both, which is what most engine builders do. Your 318 is probably at about 8 or 8.5 to 1 compression, and by swapping to 302s, you might raise it to 8.75 or so at best.

Read this article ( they do a lot of milling of heads and block, as well as utilize taller pistons ):http://www.popularhotrodding.com/enginemasters/articles/mopar/0667em_mopar_318_engine/