Bummer... I don't have a suggestion for you, except for maybe installing inserts to allow using the correct metric bolt thread/pitch. An engine shop near you might have a better solution, or be able to install the inserts for you.
And, our Troubleshooting & Repair Guide specifies what tap to use. This Guide is not very expensive... See my signature below. It contains the best info you'll find anywhere for replacing head gaskets correctly, and lots of other parts.