B body axle in A body?

B body rears fit in A bodies fine provided you get the correct offset wheels.

A 68-70 B rear (60 1/8") is 3" wider in total than an A body 8.75 (57 1/8"). A 66-67 B rear is slightly more narrow than a 68-70.
71+ B rears are even wider than the 68-70

Perches are at 44" vs 43" for the A body. I've put them in before stretching the springs 1/2" each side. It's better to cut the perches off and place them properly.

They fit in ALL A bodies, get the wheel offset correct and you're good to go.