Experience has shown that the take-apart Champion X plugs run the best, if using 1/2" pipethread spark plugs (original style).

The Autolite and Motorcraft don't run as hot and the engine won't wind out at higher RPM's.

Using the threaded 14mm plug adapters, Lang's ships Autolite 56.  Bob Jablonski has reported good results with Autolitre 66.  Another member uses Bosch platinum-tipped plugs.