I have the same problem with my foxbody coupe. I bought it drove it to ST. Louis from Florida. Everything was fine for a week at home and then it blew the first headgasket between cylinders. I had a buddy fix it for me and he ran a flat plain on the block and sent the aluminum afr165 heads to the machine shop. Everything seemed good then when he put it on the dyno it popped a headgasket between cylinders on the otherside. Now the car is in storage and I'm planning my next motor. I still to this day dont know what the problem is. It is NA too no NOS or anything to my knowledge.
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