Hello and thanks all who responded. Installed the colder spark plugs today and this has helped; also found during the swap that it was running a bit leaner than I'd thought...still a light ping is present, however. I'd describe it as less extreme than the usual marbles-in-a-plastic-cup noise--that's what it was doing when I dumped the octane "booster" into it--this is quieter and gentler, and continuous while the engine is under load and in the 2500-5000 rpm range. Especially bad in OD.

Things are improving, and timing is next on the list--but I'm going to hold off on retarding the timing further until I'm sure the last of the booster crap is burned up, so as not to change too many variables at once.

On the pistons: these are Jahns pistons, no longer made (I believe Jahns was absorbed into J&E pistons ?) and purchased from a guy who was going to use them along with a milled head in a race engine...in 1967. He never got around to it. The "dome" on the pistons is more properly called a bump-up, it's rectangular, about 1.5x3.5 inches and 1/8 inch high. Pistons are .030 oversized.

Christopher: as near as I can tell from a glance at my scrawled calculations from last year, combustion chamber is 58cc, including estimated thickness of head gasket, 55 without, which I calculated would produce 9.44:1 compression ratio...

All best,
Nathan