Help with spark plugs

[BroncoLew]

I decided to replace the spark plugs on my 76 and got the plugs from O'Reillys. I've attached a picture of the new plugs and one that I pulled out. PO told me it was the original motor but was rebuilt but I checked the block and it has a casting number of C8OE-6015A. From what I can tell its a 1968 302 but it has the Duraspark II ignition setup. Does anyone know what spark plugs I need to use?

 

[Broncobowsher]

You are correct, those are early heads that take the bigger 18mm thread spark plugs. The smaller 14mm are the newer heads.
Pretty common back in the day to swap out a set of heads with a rebuild set. So not surprising at all.

I would take the numbers off the old plug and cross reference those to the style of plug you want to run.

 

[ba123]

Yeah, it’s the head that dictates the plug, not the block.