On my '65 I measured to the inner wall around the wheel tub area. I get between 70 1/8 inches and 70 1/4 inches. There seems to be a curve there. The inner walls are closer at the top of the tubs and farther apart at the bottom. I think it would be better to use the ends of the bed sides as reference points.

I measured at the inner base of the stake pockets at the tail gate and got 64 13/16 inches.

I measured at the front top inner edges of the bed and got 64 15/16. These are pretty much the same surfaces as the inner base of the stake pockets measured above. I would expect that if other members measured them we'd see some variation there.

I would recommend that you tack everything and mock it up to make sure it all fits the way you want before you final weld. If you are going to run a tailgate you should assemble it into the mocked up bed prior to welding as well.

Let me know if I can help with anything else. edit: I hope I have described the locations sufficiently. It would really be best to have a sketch as a reference to be abundantly clear. I just don't have the tools to do that here. If you can, make a sketch so that we can be clear that the measurements are what you think they are. I'd hate for you to do all the work to build a new bed and then find out it's a couple inches off due to a miscommunication here!