It is possible to swap the crossmembers from '71-87. The most preferrable choice is the '71-72, as the rear end is swappable as well, giving you 5 lugs all around. After '72, the rear went to leaf springs and all the mounting brackets change.
I am currently writing a tech tip for this very thing. I swapped the front and rear of a '75 onto my '64 frame. Sorry it's a little late, but hope it helps.
