I swapped over from a mechanical to an electric pump on a 4.3L. To simplify install I bought the Airtex E11003, found it online a lot cheaper than through OEM marine sources. Nice part with the Airtex is it is the OEM pump, so you can use Mercruiser mounting brackets and rubber grommets and the Mercruiser brass line adapters. Even found the pre-bent stainless fuel line, useful if your carburetor is one that Merc used. Made for a very clean and very stock installation.
Rick