Just bear in mind that you have an auto box, so you would need to do a gearbox swap too. I don't think a 4.2 will do well on an auto box. EGTs will play a major roll in the limitations, and then there is the electronics.....
I stand corrected but I believe the TD42Ti motors that came in the 4.2 Patrol pickup has an ECU and EGR, so is a fair bit different to the TD42's that we have in our station wagons or like the TD42's in the old TD42 pickups like Tinus's
However having said that, the 3.0 auto box may just be able to talk to the TD42Ti ECU..... you will need to do research.
Bear in mind too that the fuel consumption on the 3.0 motor is way better than on a 4.2. Powerwise the 3.0 will have more than enough grunt and power to push the Patrol around quite comfortably, probably better than the 4.2 will do in a few instances. To get the 4.2 to be comparable to the 3.0, you will need to spend a bit of extra money for sure..... you will have to weigh up the wants vs needs scale.
I think the rebuild ( properly done ) may just work out cheaper than the transplant labour and cost of changing to the TD42. and the fact that you will need to open that TD42 to ensure all is well in there... and replace bearings and rings etc while you are at it......
Nissan Patrol GL 4.2 Diesel Turbo (Iron Maiden)
Nissan SANI 3.0 V6 4x4 (SOLD)
Isuzu KB 280 DT 2x4 ('ol Smokey) - SOLD
Suzuki TL 1000R "V twin" (Growler) - SOLD