Narrowed Rear end questions

I am picking up a 73 Duster and am planning on Mini-tubing it. I have already picked the wheels and tires out for it. I am gonna be running 325/50-15 rear tires on 15x10 wheels with 4.5" back spacing. The guy that I'm buying the car from is going to narrow an 8-3/4 for me, along with weld on spring perchs and a back brace. I know that the correct way to do it would be to have the car here and stuff the wheels and tires underneath it, then measure it that way. I just can't have the car sitting for a long time without a rearend underneath it in my driveway. Anyone have a duster/demon with a rear setup that is close to this one? I need to get a measurement from flange to flange. Thanks in advance for the help