For the glo plug shim idea, it might work but again it depends on how close the piston comes to the head. High compression engines have a tiny squish-band. So, with the shim, you lose some amount of thread engagement. Depending on how many, or how thick a shim one uses (based on need [piston top location]) one might strip out the threads because too few are engaging.

I have thought about how one might make a threaded bushing that could be brazed or somehow threaded onto the existing glo plug so as to engage the full intended number of threads, yet still have the plug sit high enough so as to not interfere with the piston top.