Richard: the Brodak 40 is, in general, not tough on plugs.
I have John Brodak's very first engine, his personal 40, engine number 1. It has been run in quite a frew different aircraft as well as a variety of props, from 10-6 up to 11-4.
I have run a bit of Powermaster 10-22 (50/50) but mostly Brodak 10-23 (50/50).
The engine has been mounted both upright and profile.
I do not know what plug is installed but, whatever it is, it has NEVER been changed.
This engine, by the way, flew the very first Brodak ARF, the profile Cardinal and now resides in the original prototype ARC Smoothie. It's still one of the easiest starting and best running engines I've ever used.
What Bill Hummel said may indeed be the solution to the problem. If the tolerances are off enough to raise the compression substantially, that could quite well shorten the plug life. Try a few extra head shims and see what happens.
Bob Z.