May have just been the last one that created the start issue. How did it run before after starting? kind of miss, or a little sluggish at first, or extra smoke? These could be signs of a cold cyl?
Changing the GP is remove wire, rempve gp, clean hole if you wish, a lot of people use reamers, a little antisieze on the plug threads only, plug in, wire on. Test again, Good to go!

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83 300SD Dark Silver Dark brown inside