Roof Skin Replacement....

I have done several roofs skins although none on a Mopar a-body. I can't imagine it being all that different as most cars were built similarly in that era. If you do the skin, think you will find it easier to panel cut an inch or two above the spot weld all the way around, remove the roof and then drill the spot welds with a spot weld cutter. Make sure you miss the roof supports. That way you can get your chisel tip under the skin to help you along with the stubborn ones and it makes it easier to stand up inside while you drill. Also, a steel wire wheel on a drill will eat through the seam sealer in the trough quite nicely. A brass wheel will work but may take longer.
If you splice the pillars, you really need to sleeve each pillar about 2 inches inside and above the cut otherwise you will not have the strength you need in a rollover situation. Where I live, it's actually illegal not to do it that way.