My built 66 L69 400 motor was tuned for around 2000' elevation and ran great. I'm now living at over 7000' and I keep fouling plugs on startup. I'm going to sell it, so I don't want to go thru changing all of the jets, etc. What is the hottest plug I can install to hopefully prevent further fouling?

 

Randy, an R47 is pretty hot compared to an R45. They may work ok but you might get a little pinging.
You may have to adjust the timing some. Let me know if you need a set.