is it worth the work

I just rescently aquired an 83/4 rear out of an 69 or 70 coronet or charger either way its a b-body rear heres the kicker its narrowed to these specs.
Flange to Flange 39.250" app. kinda hard to bend tape measure around rear leafs lol
spring center to spring center 32.000"
Now for the tricky part I have a 73 dart swinger with what i beleive to be the stock 81/4 rear measurements are
Flange to flange 51.400"
spring center to spring center 43.020"
the difference between the two should create a 6.000" app. gain in tub space am i correct in this way of thought???????
OR IS THAT JUST PLAIN OVER KILL???????