I don't think Dave still sells anything for the Nelsons, it's all handled by Mike Langlois at aero precision machine now.
Also, I don't think you can still get stuff for the SS engines (I'm not 100% on this though, I just don't have SS engines).
Standard Duty plugs are no longer available, only HD's.

Make a test prop by cutting down and balancing a 7.4" Q40 prop to about 6.25" diameter. This will simulate the prop load in flight. Use this prop as a benchmark prop for all testing.

If you really blow too many plugs (and not because you are just running the engine lean), raise the head some (if you have shims that is). Don't overdo it, since you will also be blowing plugs if you raise it too much (because the engine becomes difficult to needle, and you will likely end up too lean.

And again, a lean run will blow your plug regardless of where the head is set at. So first try a couple of flights with a nice rich mixture, and see how a new HD plug looks after that.

Also, 500 rpm rich of peak sounds like too little. I don't use a tach for setting the needle, but I'm sure I'm setting them way richer than 500 rpm rich of peak.