318 piston questions

You really need to know how far down into the bore the pistons will sit, the size of the chambers in the heads and the diameter of the bore of the head gasket and it's compressed thickness. If the "flat top" pistons have valve reliefs machined into them you will need to now the volume of the reliefs.

You can use this calcualtor to figure out what your compression ratio will be. http://www.csgnetwork.com/compcalc.html

If you are trying to build an engine for performance you should be looking to get over 9:1 on the compression.

Also, the advertised compression ratio of pistons almost always winds up lower. The reason is the deck height of factory blocks is almost always higher than the nominal design dimension.