91 Octane in a 340?

Not sure if this helps you or not, but I ran a 340 w TRW forged 10.5 to 1 replacement pistons (actual comp probably mid 9s as already mentioned) 272 455 purpleshaft 230 @ .050 or so, performer intake, headers, mopar performance electronic ignition 35 deg all in at 2500, and j heads. Engine would not run on lower than 93 octane. Hottest part of the summer I had to run octane boost or it would diesel when shut off. Admittedly I was not sharp on tuning 30 years ago. I also live it Georgia and it can get 95 degrees here pretty regular in the summer.