I think that grafting the box sides on a later model box would make things easier. It's a good idea, I used a 65 box as when I started my project , I did not have a box. One thing I did find out is if you have a side cargo box it is to wide to fit the later frame
I removed it and thats where my batteries are. I did have to wack 2 inches off the back off my frame. I just drilled new holes in the frame for the box mounts. Eric