need help with 8 3/4 shortening measurments

so im almost done with mini tubbing my 70 dart swinger and im in need of the proper measurments for shortening 8 3/4 rear end and axels im going to move the springs to the center of the frame rails my plans are to run the widest tires possible and in hopes of putting 17x12 rear wheels so for those of you who have already done so...any help with measrments would be appreciated

thanks
Dave