Swapping to an 8.75 suregrip

I have a 72 Dart and I have recently installed an A833 4 speed. The rear end I have in it is a 7.25 and I saw the post about how long the driveshafts are supposed to be but the combination of 4 speed and 8.75 was not posted. I need to know how long to have my drive shaft cut down. I also had my axles rebuilt but the man at the shop said he used "green bearings" because the old style adjusters aren't sold anymore?? So now he told me to remove a small block in the center of my third member so that the axles can free float?? I am not rear end savy so any help is lovely! Thanks in advance gents!