Blown headgasket. What's next?

Lots of engine changes and a new carb and pushing the timing up..... you could have easily been on the verge of lean on some cylinders with all the changes and the resulting unknowns, and detonated 'just enough' with the sudden load increase of going to 3rd gear. Fuel/air mix to all cylinders won't typically be the same in a carbed engines so having 1-2 cylinders 'go' and not others is what can happen.

The bent ring on the one cylinder looks suspiciously like that is what happened. I'd be pulling those pistons looking for broken rings or lands too.

Is there any odd scoring on the walls of those 2 bores, or odd pitting on the piston tops? Any recent overheating issues?