So as long as you can change the bellhousing on the 720 gearbox you should have a good chance at getting the conversion to work? I would have thought you will have problems with the licencing as the TD27 didnt come out in the 720.
If you don't intend on selling the truck, just driving it into the ground, I wouldn't bother as it won't raise eyebrows, wow a slow old Nissan diesel in a slow old Nissan truck, but if you do plan to sell or might in the future, engineering won't be too big a deal IF there's no welding and the motor is as-fitted to the donor vehicle. You might get away with engineering it for $300 if you shop around (VIC prices)