Spark Plugs and gaps

I assume youre not mislabeling the factory 351 cleveland as a windsor.The windsor series of motors has two possible plug types depending on the year the small 5/8 socket later heads(autolite # 25) and the early 13/16 socket(autolite # 45) types.gaps depend on your ignition setup 32 to 45.I use Autolite plugs # 25 in my cleveland and i run a gap of 40.Im running a mild 351 with an MSD dist. and box.Hope this gives you a starting point.

I have a very old 'Hawk' tune-up guide. I use it a lot because it covers the years 1964 to 1974. It does not suggest plug Brands or #'s but as far as Plug Gaps go: 1971 351 4v (no type,I.E. W or C)=.036", '72-'73 351 4v (no type)=.034", '74 351 Winsor=.034",'74 351 CobraJet=.034", '74 351 Cleveland=.044"-AND there is a footnote! You turn to the back of the page and read:"With Electronic Breakerless System .054", Remember this was the year when Electronic Ignitions FIRST came out. This guide also suggests testing for Spark-Plug Gap eficientcy, using a Vacuumn Gauge. The Higher the Vacuumn, the "Finer" the tune; and the greater the Gas Milage! I have such a gauge perminently installed on the rear port of an Edelbrock Torquer Manifold,(750 Holley-Vac. Sec.). I run an electronic ignition so my gaps are .054" the Vacuumn reading at Idle is 19+ 'Inches of Mercury', on a '74L Internally Stock Engine with the 8.0:1 Compression Ratio. Hope this answers some questions. Marlin.