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Hotter spark plug for my 75 318

Iridium [ & Plat ] do NOT cover a wider heat range than a 'copper' plug. Irid & Plat also use copper in the centre core.
Best thing I’ve done to my current car was to toss the champions and put in Ngk. Same problem you had with a miss on warm up. Never seeemed to tune right, plugs looked fine. Now it responds like I expect when I make a change and it runs smooth.

I used to run auto lites until they stopped making them in the US. If you go with iridium, those will actually cover a wider heat range than normal plugs. Like two to three steps rather than just whatever heat range a copper plug is made to be.
One of the best thing I've done is to toss the NGK's and put in Champion's. I like the Champion UJ-11G or Ho-8A, (realizing that these are 3/8" reach plugs, as my engine is a 440) and the 3/4" reach equilivant would be N-8G. An AC equilivant 43XLS (43XL) or 44XLS (44XL); the "-S" suffix, is the projected electrode design, will work very well......but....to each his own.....