I'm still undecided on which engine to install as long as it can be mated to the Berkley 12JB. My concern is engine placement, mount location , mount installation , and adapting the new engine to the jet.( all I've never done)

Hi all, I'm ready to begin work on my 1970 18' Sea Ray Pachanga SRX. It has a Berkley 12JB installed but the 455 Olds is long gone. The boat will be used on the E.coast in salt water. I'm considering dropping in a Mercruiser 5.7 or 383mag for reliability and better corrosion protection. I want to keep the Jet. I'm pretty good mechanically but I've never done an engine install before. Should I leave it to the Pro's or Jump right in ? What are the  challenges of converting  a 455 to a Modern Mercruiser. Thanks for any responses.