sterndrive choice yanmar V volvopenta

[Layabout]

We are repowering our 10 metre coastal cruising barge We are reconfiguring to install a sterndrive, currently powered by a 225hp 4 stroke yamaha which does a good job but we plan to cruise further around australia northern coast and a) don't like the idea of 600 litres of volatile fuel under us and b) can pick up diesel off coastal supply barges which we can't do with ULP.
The barge is aluminium construction 4.5 tonne fully provisioned and manned
shallow draft. We are in no hurry when we cruise and plan to make our travelling as fuel efficient as possible but also need to have capacity to move out of trouble in bad weather in cyclone season. the OB fullfills this but is not designed to run for long periods at lower revs
We have done some research but want input of knowledge that we don't have and don't know what to ask.
we would value any input and advice
TIA
Considerations:weight size and fuel economy at lower revs, Steyr is also a possibility but is not common rail as are volvo/yanmar has a differnt fuel delivery system ?