273 exhaust manifold interchange?

I have heard from reliable sources that 68-71 340 manifolds and magnum manifolds will not fit early "A" bodies. I have not tried them myself. I sent my manifolds, original DS and PS 73 340 center outlet, to Jet Hot for ceramic coating. I wish I had stuck with the original PS manifold. I also agree with cavemanmoron to ditch the flapper.