Dub will tell you this as well as I ......

Blown plugs come (97% of the time) from a lean run.
What caused the lean run ...... that is what you are looking for.
MOST of the time when people can not see any other cause, it can be traced to the fuel system and very likely a fuel foam issue.

A head shim or two really will not prevent blown plugs. The head spacing and glow plug combine to define the "ignition timing".