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I've found that widening the gap in spark plugs seems to make the car run better with this crummy fuel that we must use. My '69 427/435 calls for a .035" gap but I run mine at .045". At .035" the car wants to run a bit rich and just by opening the gap in the plugs, it runs like a champ. I've been doing this for almost 20 years and so far, no ill effects.
The main consideration when deciding the gap is the ignition being used, the compression ratio, and the anticipated rpm. Most electronic ignitions allow for a wider gap, which is usually benefitial at lower rpm but too much gap can be a problem at high rpm. The instructions for my MSD box recommended gaps from .040 - .060 depending on cr, higher cr's using less gap. If you don't want to experiment, set `em at .045 and you should be ok.