What were the two spark plugs? There are full threaded and half-threaded (like the Autolite 10x and 76x series, respectively). Both protrude into the cylinder the same amount since the tapered seat is the same distance from the electrode. Other than the thread, there is absolutely no difference between the plugs. They do say that the half threaded plugs are a tad bit colder since they have a better conduction path from the electrode to the seat without the threads to disrupt the conduction.
Could this be true for the plugs you tried? Look at the electrod to seat distance for both.
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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team