The timing you run is a major problem and could be a big contributing factor, detonation raises cylinder pressures by a LOT...and if the h/g is the weak link, it will blow out first....and if that holds, well, something's got to give at some point.
At 89 octane, I would run no more than 12 degrees of base advance. 15 degrees on 93 octane makes for good safe power. 20 on 89 is just plain asking for trouble. I'll bet your spark plugs look a little melted/warped, maybe even have tiny little balls of metal stuck on them. Know what those little metal balls are? THE TOP OF YOUR PISTON