There is another board member who posts here some, (bored & stroked) that has a 383 with Vortec heads. I have never seen him say anything about his engine pinging.
What I would suggest is working with your distributor weights and springs to help stop it. I am making a guess here but with the 64cc Vortecs, you are probably around 9.5 to 10 to 1 compression. You are on the edge with 92 octane fuel (with iron heads) but you should be able to run without pinging if you get the timing right.
I would also recommend using the vacuum advance for fuel economy. You can run both with a Crane adjustable vacuum advance canister and correct centrifugal settings. It will take some time to get it right, but nothing worth having is easy.
Just as an example, I know of a guy with a 454 running right around 11 to 1 with iron heads and he has no problems what so ever with spark knock. Of course it took him many hours to get it right.
A lot of the time it isn't the cc's of the heads causing the pinging. It can be too thin of a head gasket, too much quench area, too much initial advance and other things. Have you tried lowering the initial advance until it quits, just to see how far you need to go?
Just a couple suggestions.
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Mike
1985 Chevy C-10
Mike's 73-87 Chevy Pickup Site