Crank end play, destroyed converter, 408 nightmare

Check the progressive fire-ring failure pattern on that headgasket from a almost no distortion on #6 to maximum at #2. And #8 matching #4..... and all failures on the intake side. I think that means something.
I can't really see detonation on the piston tops, soooooooooo

As for heat, I would check the piston clearance, and ring-gaps. I had to take my engine apart and machine-hone it out another 3/10s, plus open the top gaps up a tad, which solved my heating issues. I run it at 205*F now.
I had a pair of .028 gaskets do what yours did, at 11.3 Scr on my 367, but with a 292/292/108 cam. The cylinder pressure with aluminums was about 180psi. I redid the decks a lil rougher, and installed FelPro .039s, with no more trouble. That was in 2001 or 2002;IIRC.lol. Mine were still sealing but it was close.