I've done this on my '68 vert. Take lots of measurements. I'm assuming you're also replacing the trunk floors....if not, might be a good time to think about it. I replaced both rear rails....the last 18" of each. I had to cut the replacement rails and recreate the flange (about 3" deep) for a lap joint. Welded both inside and outside of the joint. I have some pics, PM me if you're interested.
As for a spot weld bit...I've used the Eastwood bits...wore out pretty quick, but that could have been my fault. I recently bought a spot weld cutter, looks like a wood hole saw, and it has worked beautifully. I have removed over 150 spot welds in replacing my cowl pans. It is a double sided bit. If you're not careful, you will break the bit, so go slow. I bought it from my local auto parts store for about $25 with 5 replacement bits. It should last me a long time. It takes some practice and it cuts really quickly.....so be carefull. Good luck,