Hello TFISH and welcome!
Here is info out of the archives courtesy of Pete Chronis concerning this issue.
How about some photos of your family truck?
Quote:
Steve, Absolutely do not use the longer reach plug in your engine, earlier V6
engines had the shorter reach plugs. The AC spark plug number for your 305a could have well been an AC-C44-S which doesn't exist anymore. From what I could figure out checking the tune up specs on these engines is GMC switched over to the longer reach plug in 1963 and used them up to 1972. In 1973-74 the 379, 432
and 478M engines got new tapered seat plugs with much shorter reach than what you have. In my collection of five V6 engines four have the long reach plugs and only the 1973 478M has the short tapered seat plugs. I have a 66 351E that has the Autolite 65 plug which is one step below in heat range to the 66 plug and it is the long reach plug. You may not like Autolite but it is a much better plug than Champion plugs so your choices may be limited here. Pete Chronis Northern Cal.
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