Making sure that the carpet isn't in the way, get yourself a nice 5/16″ drill bit and go to town. Along the way it's wise to temporarily put bolts in place to ensure nothing shifts while drilling.
Inevitably, you'll have a few rough edges after drilling that are always annoying…
… which is when a deburring bit comes in handy. This also makes applying touch up paint a breeze to ensure rust is held at bay.
Much better and nearly ready for the install!
After delicately smashing the hell out of the frame rail back into, more or less, the right place, all the rust was removed with a wire brush. Everything was then cleaned up and resprayed with underbody coating.