I build engines for a living and have a lot of experience with Olds and Pontiac 455's. First determine the heads you'll be using, then the compression ratio. For the Old's 455's there are a lot of piston choices and most are dished so you can put the compression pretty much wherever you want it. I shoot for 10 to 1 with most of my street 455 builds with iron heads, some go much lower. Depends really on the goals for the project, power level desired, etc.....