Years ago I had Cometic make me copper RD400 head gaskets in various thicknesses that have a 66mm bore. The most common used number is for .020" thickness. This part number is H0388020C , if you need a different thickness check their online chart for copper thickness and replace the 020 in the number, for example 027 , order part number H0388027C etc.
John, see my rebuild thread. First startup will be posted soon!