When you switch to manifold vacuum, you should set the timing with the vacuum connected to the distributor. And then set it a little high, like 12-14 btdc. You were probably running 18 or more initial if you didn't set the timing with the vacuum advance hooked up when you were using manifold vacuum. And if it's pinging and you are having problems with idle in drive, you may have a miss issue or a faulty module if you're running an electronic ignition. Maybe combined with a tight torque converter or worn carb body. It's a system and it can get complicated when there's a problem in that idle area. A lot of factors working together and if there are multiple problems with the combination you can fix one and see an improvement, but it still isn't right. And you can chase your tail if you don't look at the whole picture.