I am attempting to sell my 71 chevelle rear local here in Peoria Il.
I'm not an expert on what year car this will fit into.
My basic understanding is that it will fit 68-72 Chevelle/El Camino/A-body.
I have a question with someone asking if it will fit his 66 ElCamino. I think it will be an inch too wide, but I thought I'd ask the experts.

You are correct, about a half inch per side.

Its wider, but it will bolt in. My brother has a 70 Chevelle rear in his 67 Chevelle.