New to rear end swapping

Hello folks, so looking at a 742 casing rear which I believe is an A body rear, spring perch measurements are 43.5 from the looks of it and flange to flange right at 55. I want to swap it in my 1974 plymouth valiant 4 dr sedan. I know I'll be looking at shortening the drive shaft or getting an aftermarket but will I have to reset spring perches and do pinion angle? I was looking at shortening it and putting axles for bbp.
Thank you