Don't know of anyone personally but I would suppose there would be some weight savings involved and I believe some of the internals interchange between the two such as clutch packs and some others? Don't quote me here it's been awhile since having one apart but this seems to be part of the info clogging my brain still!
They are supposedly just as bulletproof as the 727 when done correctly. If my oldest boy ever decides to build his Duster up I'm going to research it just to see the cost differential and if they truly can be as reliable since it's a slant six with the 904 and he's wanting to drop in a 360 crate eventually.