Adam I don't believe that's correct. I could not find a good photo of a B body small block with manifolds, but I don't remember them wrapping around the steering like the A bodies do.
In fact, years ago I put a 340 into my 70 RR, and temporarily got a set of 318 manifolds and Y pipe from the junkyard, until I could afford headers for it. I'm sure I'd have used the A body exhaust if it would have fit.