Compression Issue (thin head gaskets??)

I recently did a similar build. Stock 318 and bought a set of #302 heads that were shaved down .35 Also had a 3 angle valve job, cut back the valves and port matched both sides. Machine shop did a chamber test and I'm right at 56mm. Was going to run the thinner head gaskets but decided spending $80 on a set wasn't worth the extra 6hp.. lol