8 1/4 swap questions

So I'm gonna take out the rear end from my 78 b100 to put into my 76 dart sport. It's 58" wide and the 7 1/4 is 52" drum to drum.
Seems like it might just fit under there and put the wheels where they should be. Am I wrong about this?
I'm hoping to just cut the old mounts off the old one and weld them onto the new one.
I'm sure I'm gonna have to change the length of the original drive shaft and maybe have a diff spline put on to fit between the slant six 833 tyranny and rear end.
Anything else I'm missing?