After cutting out the rust on my RH front frame rail, I ground around the cut, gave it a coat of Ospho, and then a coat of red oxide ZeroRust that I picked up at APS Paints for $57 a gallon. The Ospho converts the rust to iron phosphate and the ZeroRust sticks to that and prevents water and air from activating the rust in the future. I also painted the patch inside and out with DP74LF primer.
After another hour or so of adjusting and fitting the patch (funny how you think you got it just right and it still doesn't fit when it comes time to weld) I welded the seams on both sides and ground them flush.
As a side note, I discovered that pre-painting butt-welded patches like this is probably not a good idea since the paint all around the weld joints catches fire during welding and needs to be repainted afterwards anyway.
Finally, I repainted the entire work area again with a coat of ZeroRust and test fitted the new frame rail after surgically removing the remainder of the old rail. I left the lower seat reinforcement in place for this to help me align the new rail section.
I drilled 14 holes on each side and another 10 holes on the bottom and then plug welded the frame rail extension floor support to the rear portion of the front frame rail such that it contacted the floor reinforcement box and the tunnel crossmember.
Finally, I ground the plug welds down and will paint over the bare metal to complete the task.