IF you get it to run at all you will suffer in performance, the shorter plug will result in less compression but won't hurt the engine. If you normally ran short plugs and put a long plug in, you would damage the piston and or connecting rod.
Get the right plugs for you mil and sell the other one to someone that can use it.
Could be worse; I run "turbo" plugs in two of my engines; and not one of the three shops in the area (had four, but the track closed... ARGH!!!) carries them, so I have to mail order 'em in bulk.
Besides, this is that excuse you've been lookin for to get that new hopup part you've been eyeing