Timing Question

It seems like you have way too much mechanical advance coming in, that's why it is pinging under load. 12.5 degrees initial is pretty mild, my 360 likes 18 degrees, so for your engine you are probably right on the money for the initial.

The key is now to limit the mechanical advance so that you have a maximum of 34-36 degrees timing total timing (initial and mechanical but no vacuum advance). So if you have 12.5 initial then you need a max of 23.5 degrees centrifugal (mechanical) to get the total of 36 (again with vacuum advance disconnected).

The initial and total timing (initial + mechanical) work together as a team. What is happening with your setup is that when you set the total timing (no vacuum advance) to 34 then you won't have the pinging but the car is a dog off the line because the initial timing is around 0 degrees.

When you set the initial timing to 12.5 the initial timing is perfect but the total timing is way too much, creating pinging on acceleration. You'll need to set the timing curve back to stock (springs) and check what the number on the mechanical advance plate is.

Most likely if the distributor is stock it will have the number 11 on it, this is crankshaft degrees so it would put double that in as mechanical advance (22 degrees) 12.5 degrees + 22 mechanical is 34.5 degrees total without vacuum advance. Right on the money.

Once the initial and mechanical timing is correct then you can add the vacuum advance to help out with highway fuel economy. You'll probably need to adjust that to limit pinging on part throttle. I can help you out with than once you have the basic timing sorted out.