We did this on the same truck you're working on:
http://67-72chevytrucks.com/vboard/s...d.php?t=786335
Nothing magical about it. Like any body panel fitment, you're going to install the box loose and dial it in. There are always compromises with reproduction pieces which require more massaging and fiddling. Of special note here would be tailgate and splash guard fitment. We ultimately used OE splash guards. Besides the shut lines, tailgate action can be very difficult to get just right...so it takes time. One minor hack on this that you might not think of, is to shim the rubber tailgate bumpers. You can get washers of the same diameter (so they're practically invisible) and put one or two under the bumpers to advance the tailgate contact and provide extra traction and minimize the chance for paint contact. It made a difference for us.
Otherwise, just start wrenching it!