First off, if you have to move the perches and reweld them it is not a "direct" bolt in. The drive shaft will have to be cut no matter what, if you are going from a 7 1/4" to an 8 3/4", 8 1/4" or a 9" Ford, all four of them take a different length drive shaft. The 57-59 Ford rear is a "direct" bolt in, no welding needed, it even excepts your original e-brake cables.
The B-body rear housing is 2 1/2" wider than the A-body rear, (I have both sitting side by side behind the garage) and if you have a 67-69 Cuda or a Duster/Demon body style then you can move the perches in and use the B-body rear without too much trouble, but if you have an early A or a Dart (MoparMark has a Dart) then it leaves you with very little tire/wheel clearance and practically no wheel options, the extra money you spend on having special wheels made could be used to buy the correct rear and have less work involved and leaves you with more wheel options.
I've seen numerous people on this site post the B-body rear end option and say to just push the springs out to make it work, that is a hack job way of doing it, I've only seen one or two post where people actually suggest to move the perches in which would be fine in a Duster/Demon or a 67-69 Cuda but not a Dart unless you really like the goofy look of the wheels you will have to run.
It is just my opinion but if you try and shove a rear that is that much wider in a car with hardly no wheel clearance to start with just to save the extra money to buy the correct one, that to me is a hack job. I could see that when we were 18 but as an adult.