You probably have a good reason for wanting to do this but the Atomic 4 is a great engine and parts are readily available as I assume your engine has mechanical problems. You should pop on over to the C27 national site and check with those guys.
The rear end of the Catalinas are not designed for mounting a OB. In other words, you would have to do a lot of prepertory work to beef up the stern area before attaching an OB. A dock neighbor of mine completed a conversion such as you are considering on a Newport30. After watching what he went through it is not something I'd mess with.