440 timing pinging and popping thru exaust

It does sound like there could be a secondary issue going on. On the one hand you'd expect the pinging to go away when you tried retarding the timing. - But when retarding the timing is causing it to backfire through the exhaust it would seem as though it's retarded too far. Maybe it's just a coincidence that a second problem showed up at that time but most people don't believe in coincidences. I think I'd double check the condition of cap and rotor first. Rusty's suggestion for timing should be in the ballpark for where you'd want to be. If you're running a vacuum advance I'd also suggest making sure that the vacuum line didn't crack when the distributor was being rotated. I'd also see if the advance plate is binding. You'd said that it backfired through the exhaust during WOT after you'd retarded the timing so maybe the distributor isn't advancing your timing anymore as RPM increases.