The Olds 403 is related to the other "small-block" 64-up Olds V8 engines: 260, 307, 330, and 350. They all share a common 3.385 stroke. 403 has a standard deck and a huge bore, cylinders are siamesed together so make sure you've got good flow in your cooling system and radiator. Biggest bore of any small block ever!
400, 425, and 455 are considered "big-blocks"
I don't see any reason why you couldn't add a Caddy Seville PFI to it if you wanted fuel injection, just pick slightly larger injectors.