The best way to establish rear end width is to get the wheels and tires you want to run. Say 12" wide wheels with 14" wide tires. set them under the truck with a min of 1" clearance to the springs, frame or what ever they may rub. Now measure the distance from between the hubs. that is the flange to flange width you will need.
It is easy to narrow the rear frames on these trucks, approx 4 to 9" with stock springs and it does not require any special jigs. It will align itself. I did a 72 Luv for a friend a couple of years ago. To see one done goto
http://www.photobucket.com/V8Courier there is pictures there of a courier and the Luv can be done the same way.