Hi, That looks good and is a slightly more compact version of the thing I posted earlier. http://www.veth-motoren.com/vervolg_full.php?pagina_id=101
LOL MY Welding skills are excellent but honestly that would seem to be a rather small part of this idea. In my younger days I worked on fishing boats of about the same size as this one they ALL had single inboard so it seemed to me that a I/O unit would be better maneuvered than they were. LOL our neighbors never cleared the docks even when that dumb 19 year old kid (me)took them out. Why the single diesel? Its right there in the OP. Why did I ask the question? Because I didn't know the answers. Those that have been helpful with answers and valid suggestions I thank you. I must particularly thank those of you that pointed out pitfalls. While I'm very comfortable with diesels and machinery construction I'm not experienced with marine construction or the reasons for doing things a certain way. You others? Well you know what you are.
With the type of boat that you have, a single 6v71 would be your best bet if going the single route with a conventional straight shaft. However, you could rebuild what you have and be much cheaper then retro-fitting a single......Or go with 2 small diesels in the 200-250hp range....or sooooo......