^^ Good write-up, have not read the entire thread... hopped on at the end. What kind of fuel was in use when the MP timing spec was made? When I was in CA I found more timing all in early worked better with the crap gas... especially the "summer blend".
I'd have to dig out my old notes. The timing was measured on my 340 with Comp cams 280 around 1995 - so NE reformulated gas, but maybe after MTBE was thankfully banned.
The timing shown above in blue for joepole is that advance curve shifted so the timing at 2800 rpm is 32*. That's the big circle and comes from the MP recommendation for that distributor when used with a later LA engine. I thought that might be another useful baseline.