Stripped petcock to tank screws

I want/need to work on my gas tank and was trying to get the petcock off but the PO completely stripped out the screws holding the petcock to the tank. It makes me rather nervous to drill the screws out as I don't want to drill to deep and into the tank. How deep/long are these screws? Is there a better way to get them out besides drilling them?

 

I picked up a bolt extractor set from harbor freight and it worked very well to get the screws out of my master cylinder, which were pretty small, especially afer busting the head off of one of them. And they were only $8.99 as opposed to Sears $30 set. http://www.harborfreight.com/cpi/ctaf/d ... mber=40349 The Sears set looks like a more effective design but so far the harbor freight set works really well.

 

I don't remember if they're countersunk. If not, just grab the heads with locking pliers.

If they're countersunk you only need to drill deep enough for the head to break off. Then you can grab the shank with the locking pliers.