I have not done the actual install yet (lots of body work just completed) but my 5A580 with the 5.7 attached is sitting nearby and I can tell you that it is not as wide as the 727 at the bell house but it is pretty fat for most of it's length. I saw pics of a floor mod done in a 65 or 66 Fury I think it is and it looks like the floor will need to be widened not any taller about back as far as the shifter.
Try forbbodiesonly.com 62-65 Mopar forum
Look at the thread titled late model hemi into earlyb bodies.