im another vote to change the gears , they are killing you .
also as much as you say its not pinging .... drop timing back to 11 or so , make a run . add 1 degree , make a run etc etc . watch the mph , when it stays the same or drops , go back to the previous timing and leave it alone .
i also keep a spare plug in the car , shut it off after the traps , and when on return road pull number 5 plug , put spare in and go to pits .... read the plug carefully for heat range , fuel ring and detonation .... tune from there
sounds like there is a bunch left in the combo
347 , small valve eddy rpm's , n-71 , rpm2