I took a self-tapping screw about 1-inch long through that bracket into the frame. First I lagged it into the frame before I put the pan in. I drilled a 3/4 hole in the new pan right over where it was in the bracket (socket can then be used to remove). When welding was all complete I removed the lag screw (so I could pull the cab)and covered the hole with one of those chrome push plugs from Home Depot. It makes it all come out nice and level afterwards.
You can see the plug just ahead of the battery cover.

http://community.webshots.com/photo/2273388460102454676aglcki