Pinging can be fuel mix as well. There's a few things you can do. A good start would be to pick up a narrow band oxygen sensor, probably get one or two from the wreckers for not much, they're pretty cheap new as well. Then head to Jaycar and pick up a exhaust oxygen display kit, was around $15- last time I looked. Weld a bung on your exhaust and you've got a cheap way to check the fuel mix. Or you can get a wideband with a guage for around $200-.

You can make a bung out of 3/4" (~16mm ID) water pipe. Cut to length, grind the casting flash off the inside, grind to fit the ex-pipe just after the flange and cut it with an M18 fine thread. Then weld it on.

While you're stuffing around with it setting up water injection is a good way to protect your engine (or leave it on to allow bigger boost). There's a few ways to achieve this, the simplest is a sealed bottle that uses boost pressure to push water into the blower inlet. You want to flow around 70% of fuel flow. Google will come up with plenty of different water injection methods.

1- make sure your fuel's right
2- if it's still pinging even with the timing wound back you'll need less boost, water injection or high octane fuel

Get all that right and it should be a hoot!