You should know better than anyone here, being you have both parts on hand and can measure yourself. I had both cars myself a few years ago and I think the 70' may be a little shorter on the frame rails. I would say let the frame rails tell you where to cut, but keep in mind that you will need the side lights in your cut from the 70'. There will be a lot of changes in the front also, hood fenders grill and power to the side lights, front and rear. Hope you saved your rear wiring for the tail and brake lights from the 70'.