Midwest Military just announced they had spark plug adapters made for M series trucks. You use a civilian plug and place the adapter over it. On the surface, it looks like the original plug and uses the M series wires.
From their ad;
FINALLY! 24v spark plug adapters are in! No more need to find the now non existent 24v plugs. Just get a set of our adapters and you’ll be able to run over the counter plugs for the life of the truck.