might be worth calling Castlemaine Rod Shop, they make a amazing number of conversion kits. you could also try and find something like an owners club or even talk to a Nissan commercial wrecker, sometimes they can be a mine of info on what goes with what because they see so many parts. if you dig enough you'll find someone, somewhere that has done it before.
alternatively, once you've calculated the time and money, and then doubled it, consider buying another ute unless this is something you really want to do.
google is you friend. $1400 for the V6 kit:
http://www.v6conversions.com.au/nissan720.htm