Spark Plug Differences and Recommnedations
 

I note that Wayne says in his book that Bosch WR5DP are what the 1974-83 stock engines takes.

What is the difference between the WR5DP and the WR6DP plugs, and when would one use the 5 and when use the 6?

Any recommendations?

For Bosch plugs the higher the number the "hotter" the plug (For NGK it is opposite the lower the number the hotter the plug). That said, in general use a hotter plug if you are getting some plug fouling, especially important on a carb'ed car but I see you have a CIS 83 SC. My geuss is that you would not be able to tell much difference between a 5 and a 6...but heck just try both and see how they work. Run them for a half hour or so, cool down , pull a plug and check the color.