DMax is based on nothing.
Clean sheet of paper on an Isuzu drawing board.
GM 6.2/6.5 Diesel was NOT derived from any gasoline engine.
It was a ground-up diesel design, as lessons learned from the hideous SMALLBLOCK-derived 5.7 Diesel proved to be damaging to GM's reputation for a number of years. While it (the 6.5) was a diesel design, it was not particularly spectacular in any one category other than to say that it is a good dependable unit that is far better tha it is given credit for - it was never a monster powerhouse, though.
There are already marine exhaust manifolds available for the 6.2/6.5 from at least two sources.
AND
I have sniffed rumors of a newly developed LLY Duramax Marine-Spec unit.
I'm not a member of the pay site of "the Diesel Page" so I can't view it.
Her's a link:
http://forum.thedieselpage.com/cgi-b...c;f=3;t=007996