which cam?

[Three Peaks]

Sure sounds like a compression issue to me, possibly along with the cam being installed wrong.
Sounds suspiciously like somebody built this stroker with flat top or domed pistons, giving you way more compression than you can deal with. Brent is absolutely right, you have serious problems and they are being masked by your mechanic, use of race gas, and other band aid approaches.
My 393 stroker runs 10.5 compression and runs fine on 91 pump gas and I'm running a 750 DP Holley. Running the 294HR cam with Vic Jr heads, which flow approximately the same as your AFR's. About the same hp/torque as yours and no pinging problems at any RPM.
If it were me (and glad it isn't) I would pull that motor, pull the heads and see exactly what you have in the engine and go from there. A few measurements would probably help- deck to piston height, valve to piston clearance, piston dish measurement (if they are dished,which I doubt at this point), combustion chamber volume, and just about anything else Brent suggests. A good third party mechanic would be a good idea about now so you know he doesn't have an axe to grind when he inspects the parts and build and can recommend changes without bias. Hopefully one who is familiar with strokers.

With the build you described, this thing should run great and have plenty of horsepower, no pinging, and do it all with a 750HP carb or at max a Quickfuel 850 and all on pump gas.