I've used shorty (OS) plugs to help reduce the compression ratio in engines that use long plugs. Havn't done it the other way around. I would check to make sure the long plug doesn't hit the piston on an engine designed for a short plug. Even then, you might have hot spots on the protruding threads of the glowplug and that could cause running/tuning issues.

By the way, a lot of people recommend the MC8 for 20% nitro and the MC9 for 30% nitro (in big block engines). I can't say if this is correct or not, but you might want to do a SEARCH and see what others say.