Big Block muffler choice

My FM's barely fit under my b-body, with a 22"case (thick and fat) but the 3" magnaflows are much thinner, smaller and lighter. I think they are 20"case (I'm a couple hundred miles away), and I can't tell if they would work on an a-body. Mine are hooked to fenderwell headers and run next to the rockers. Pic is the 3"magnafloww, and 3 1/2 aeros.

20200302_121217.jpg