Blown Head Gasket/Torched Head

[Chief]

Last weekend during my first time trial, I got to the MPH light and the car laid down like it went lean. I first thought I had a broken rocker but upon further diagnosis I found I had a blown head gasket between 4&6 cylinders.
This is a 468 ci Mopar. Injected/alky, Indy aluminum heads. I have been running this combo all season with no problems. I also have a qdata data logger and from the launch the left bank were running at 1040 degrees. The right bank never went above 800 degree's except for # 4 cylinder which started at 450 degrees and went to 1290 before I lifted when it laid over.
I am getting the head fixed but I have checked everything I can think of without any positive results in finding what caused this to happen.
The only change I made was when the ignition timing changed. I have ran 34 total timing all summer but I was retarding the timing 4 degrees on the shift. This weekend I was retarding it 6 degrees in low and going to 34 degrees in high.
I have checked fuel pressure, injector nozzles, filter, also the head gasket does not show any discoloration like it was allowing anything to pass.

Any ideas?

Thanks