Have the heads milled to turn them into closed chambers, that would give you about 2 full points. It would require excessive milling though (.120 thou or so), but you can get iron heads for a dime a dozen nowadays. I would have been worried about doing this, but I'm finding out that people used to build their race 383s/440s like this often before any aftermarket heads were available. I've never tried it but I've got an okay set of 906s that can get by with a valve lapping that I'm saving for something like this when the time comes.

Or have them shaved the factory recommended max of .060 thousandths, that would give you about a full point.

Run steel shim gaskets too.