My 36 ford flat head v8 has Champion (6) D16 and (2) D21 plugs in it?? It did not run when I got it, so I am ready to try.

The shop manual claims "18" champion. No alpha identifier. Just straight "18".

non of my local auto zone or advance auto have a conversion for "18" or a 1936 v8 flathead.

They do have the d16 or 21. so what do I use???

or is there another plug all together to use???

My Champion book calls for UD16