I have less than 10 miles on my rebuilt 340 in an A-body, and it is pinging like crazy. I have it backed down to 28 degrees total timing and it still has a very slight hint of pinging. At 30 degrees total and above, pinging is quite noticeable.
Here's my specs:

340 stock stroke with 10.5 TRW L2316 pistons,
2.02/1.6 stainless valved, iron heads with pocket porting and a small amount of port cleanup.
Air gap dual plane intake with a 600 holley vac secondary which may get replaced with a 750 vacuum.
1 5/8 Hooker comp headers with full exhaust
older Mallory unilite distributor, all timing in by 3000
After cc'ing the heads, static compression figured at 9.5:1

I am running an 80's version of the MP 280/.474 cam, which may have different timing specs than the newer version (different part number). I seem to remember that this cam would "build" compression due to its valve timing, but its been a long time and my memory has seen better days. I ran this exact combination in the late 80's and it ran awesome. I guess gas quality has come WAY down in 20 years.

We ran cranking compression on two cylinders and they came in between 170 and 195 psi. I'm not sure if those levels are too high for 91 octane (best pump gas I can find).

Any ideas on how to eliminate the pinging so I can crank the timing back up to normal levels? This thing is a pig at 28 degrees.

I was thinking of a cam swap to lower cylinder pressure, but am wide open to other ideas.

with 9.5 to 1 comp it should be fine with premium fuel. Sure you have good gas? Sounds like a new build. Try getting some new gas from another station. bad fuel is not uncommon

anything above about 165/170 lbs. clyinder pressure is playing with fire on pump premium.

What is your jetting in the 600?
What is your initial timing?
Are you running vacuum advance? If so, how much?
What plugs (heat range) are you running?

with compression readings that high you have an actual CR of over 9.5 plus no quench. You'll need to proportion in some race gas asap, pinging does alot of damage in addition to killing performance

Get 5 gallons of 110

Might get away with that C.R with magnums.But the original poster has a slightly domed piston (TRW 10.5) and open chamber heads.I have run the exact same set-up and detonation was a major factor.With that combo as the original poster stated backing off the timing will result in poor performance,get the timing where it needs to be and use a higher octane fuel.

""I am not sure offhand on the jetting""

That is where I would start. A lean carb will add pinging. Let us know what jets you have.

Initial timing needs to be at least 15 Degrees BTDC. The rest of the curve should come in slow till all in at 2600 rpm. Total of 32.

Distributor needs to be check for proper phasing which has been covered before and beaten to death.
A properly phased/curved distributor is required or just keep chasing your tail.

Antifreeze should be 60% water 40% antifreeze
195 degree Thermostat

Quality spark plug wires like Firecores.

Once the initial timing/recurve/phasing and mixture adjustments etc have been resolved you can adjust the jetting.