1970 dart reared question

as 72blunblu mentioned, the earlier B axles do fit but they are a a little wider then the stock A body 8 3/4

moving the leaf springs in does relatively little too, so really, if you want to run a wide tire you're looking at mini tubbing it
(of course, then you're still moving the leafs, but my point is, by ONLY moving the leafs you only gain an inch or so, and the next limiting factor is the factory wheel well, as ATA mentioned)