I think you are on the right track. I think the xe274 would be a great choice. To me the cam in the impala sounds a little bigger than the xe274 but if it is it should be a little more choppy in a small block, and still make excellet power. I would probably go 3.73 on the gears but it depends on tire size. As far as carb, if you can get a q-jet that is right go with it, if not go with a holley 750 or street avenger 670, but you either way you will be happier with vacuum secondaries.
I debated on the xe274 for my 383 but I am making 11.0:1 so I went with the 286h magnum. It sounds about like the impala clip.