What should my timing be set at to stop pinging at wot

I'm going to go out on a limb and say you have a vacuum advance that needs adjusting and you should look at your timing curve. And I would do it, rather than try to use manifold vacuum on it. On the vacuum dashpot on the distributor... Take the hose off, and slide in an 11/32 allen wrench. You should feel it engage a hex inside the dashpot. This is the preload on the dashpot, and is almost never perfect out of the box. If you turn the wrench clockwise, the advance will occur with less vacuum applied. If you go counterclockwise, it will take more vacuum to occur. While at WOT there should be no vacuum, if the carb is small, you may be pulling some at the carb, causing the advance to come in and ping. By tuning that advance, you get the performance and mileage. Vacuum advance means it's also advancing at idle and possibly when it's turnign with the starter and trying to start.
Last, find out what rpm the mechanicla advance comes in, and when it's all in. With a stock type convertor you will want to have more initial, but have the mechanical come in slower, so the total advance isn't reached untill closer to 3K, rather than the normal "2500" deal.