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.