I don't know how yours is set up, but on my 550 I've used regular feeler gauges that can be purchased at any automotive parts store. I had to bend the ends of the feeler gauges so that they'd fit. Remember after you've set the clearance, you need to slide the next size up feeler gauge in to test your work. If the next size up doesn't fit, you've set the clearance right. Checking your work will greatly reduce engine backfiring...trust me.
The only other tools I use are a socket wrench to get the cover off, and a wrench and screwdriver to set the clearances. Make sure the screwdriver fits tight in the slot as well.
These are all tips I've gotten from this forum by the way, and they all worked great for me.