I think you're saying the 4-door is a 65. I don't know what additional problems that will cause. I think Darts were the same from the doors back between 65 and 66 except for the tail lights. I don't know if the same is true for Valiants.
Did you get a title with the 4-door?
I did my entire floor pan with just one of the Harbor Freight spot weld cutters. They're only $5.
http://www.harborfreight.com/3-8-eighth-inch-double-sided-rotary-spot-weld-cutter-95343.html