As a general rule of thumb, 9.5 is a good cutoff for iron heads. 10 to 1 or maybe just a hair more is fine with aluminum heads. 10.9 to 1 is definately way up the scale. In order to keep this high of compression you really need to lower the timing. 12 BTDC is going to be too much with your compression.
Also, if your springs in the distributor are letting the mechanical advance start fairly soon (which is good normally) it will help cause the pinging. Try lowering the initial advance to about 6 degrees and see what it does. Then you need to buy or borrow a dial back timing light to check your advance. Then you will be able to tell when your mechanical advance starts and when it is full in.
When does it ping? All the time? When cruising? Hard acceleration? If in the cruise range, you can overcome this. If all the time, it will be real hard to fix.
Just so I will know, what is your deck height? What is the compressed head gasket thickness? Flat top pistons or what?
One thing to check. Remove a couple spark plugs and see how they are running. This is always the best sign of what it is doing. While on the subject of plugs, have you tried running a cooler plug? If your plugs are too hot for your setup, it will most definately help cause pinging.
Let me know the answers to the above questions and maybe, just maybe I can make some suggestions to help. With 10.9 to 1 compression, it is going to be real tough to fix it.
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Mike
1985 Chevy C-10
Mike's 73-87 Chevy Pickup Site