Think I finally figured out a spark plug that's a heat range colder than an NGK XR5 (or Champion J12YC.) They don't make it easy to do that. Ended up about the only plug I came up with is an NGK BR6S. It's a heat range colder than the XR5's in there now. All the other plug manufacturer's I looked at didn't make a plug that was colder and that would fit. I looked at their heat range chart, got the range number and then searched for a plug, and came up zip on all but NGK so,,,,,,,,,,,,,NGK it is I guess. The BR6S is the same 14mm thread and 1.25mm pitch, same .380" reach and same hex size. Funnily enough, that plug is for, among other makes engines, the '48-'68 Chrysler 230 cid flathead six. Whod'a thunk it?
Also got some 110 race fuel coming. One of the guys at Advance Auto Parts next door to work said he was going to Oshkosh this weekend and passes right by a station that sells 110, and if I wanted him to pick some up for me to just say the word. I said "word" lol!
Then all I need to do is change the plugs. Driver's side, not a big deal. Passenger side,,,,,,,,,,,well,,,,,,,,,,,a picture is worth a thousand words.

The biggest trick is getting the passenger side access panel off without scratching anything. It must come off or no spark plugs, lol.