Typically - not always, but typically - backfire in the intake is a lean condition or some condition with valves/springs. Hitting a "wall" at 3500 could be both a weak spring issue and/or fuel delivery problem. 110 is right on the edge for 12.7 IMHO and per VP. Like Gerry says, check your max advance with the 20 initial. You can run as much initial as you can stand, it's the max that causes issues.
Exhaust pops are lean conditions in the exhaust - gasket leak, loose bolts, etc. Especially if it pops on decell, like off the throttle in the traps. If you lift off and it pops through the exhaust it's almost guaranteed that's an air leak in the exhaust, usually at the header flange.