8-3/4 questions

Got an 8-3/4 rear to install in my 66 that has 11x2.5" brakes, 742 case with 2.76 suregrip and is 56-1/2" flange to flange. Whan I have it narrowed, I will tell them to make it 51-1/2 flange to flange. Sound right? I also will just purchase a-body axles for it. Does anyone know how long the a-body axles are in case I have to have them made? I posted this thread on BBD yesterday but no one will reply; must have pissed them off or something.