Putting a B Body 8,3/4 rear end on a A Body

Can someone tell me what kind of back spacing I'll need if I convert my 1971 Valiant's 7,1/4 to a B Body 8,3/4? My brother gave me his 3:23 posi unit that was out of his Coronet. I was planning on going to big bolt pattern in the front with disk brakes and the factory Rally wheels all the way around, but I'm not sure of the spacing issues.
Thanks