B Body rear into 67 cuda fastback

the rear I have for dad's car is a 64-67 and measures 59 1/2 inches from wheel flange to wheel flange.although I could pry apart the springs to meet up with the perches in their current location I'll be moving the perches to be where they should be,why have the springs in a bind???pop's wont be racin it but i'm sure he'll appreciate the job done well. the 68 to 70 rear for b bodies is 60 1/8" and would probably rub the wheel lip's is why I bought the earlier rear and it had a powerlok in it to boot(pop's wont be needin that either)