Your timing issue could be in the distributor but it could also be in
the thrust plate between the cam and the timing gear. If it is worn thin,
the cam can move endwise which also affects distributor gear indexing,
about 1:1. Sort of like a worm gear drive where in addition to turning the
worm gear you are also moving it endwise.

I'm not sure what the problem is but you should NOT have to be retarding your
timing to reduce pinging! I use the cheapest gas I can get, have carbon
buildup and run 21-24° initial advance and only get pinging rarely. And
anyone who knows me will tell you about my lead foot. Wish you weren't out
there in the far corner of the country! (But then there are folks in your
area who know a lot more about Volvos than I do!)

BTW when grandson Ed visited from Tennessee he was complaining about grandson
Ben's driving. Susan (their mother) told him "He learned it from Grandpa...."
--
George Downs Bartlesville, Heart of the USA!