Running old LA style heads on a magnum block

The problem is I built my 408 with a magnum block and I used the Icon 744 flat top pistons. The compression ratio with these pistons at 62cc is 12.1,68cc is 11.3 and 72cc is 10.8. The Edelbrock magnum heads have a 58cc chamber so I would be pushing around 12.9 and that aint working with pump gas. I have some edelbrock 60179 open chamber heads that are supposed to be 65cc and If I use some really thick head gaskets I might be able to get them down in the 10.8 range. The pistons are not -0- decked but they are not very far down.I will get a measurement today and do some calculating. This thing might end up in a race car before its over!