If you want to be original, here's what works according to the FACTORY SERVICE MANUALS
All Frazers - Auto-Lite A-5
Kaisers - 1947 into 1951 model year - Auto-Lite A-5
Kaisers - later 1951 models - 1955 - Auto-Lite A-7
All Henry J - Champion J8 (as supplied from Willys-Overland)
Kaiser-Darrin Sports Cars - Champion J8
Willys Vehicles 1946 into 1950's - Champion J8
According to Kaiser-Frazer Service Bulletin #302 dated August 31, 1951 "Under extreme driving conditions such as sustained high speeds, the Auto-Lite A-7 Spark Plug may be too hot, in which case it is recommended that the Auto-Lite A-5 Spark Plug, Part Number 201230 be used in place of the A-7". Another part of the same bulletin "While the Auto-Lite A-5 Spare Plug has a satisfactory heat range for more areas, it has been established that this heat range is particularly suitable for cars with over 5,000 miles of service". Lastly, "Dealers are urged to refrain from removing Auto-Lite A-5 Spark Plugs from new cars in favor of the Auto-Lite A-7 heat range unless it is established that satisfactory performance cannot be achieved without the use of a hotter Auto-Lite Spark Plug. Present A-5 Spark Plugs now in service should be properly cleaned, regapped and re-installed as an established service practice".