When I did the 3.3 swap on my Pathfinder, I swapped the intake plenum, distributor, exhaust manifold and I think the coolant pipe under the intake manifold from my original 3.0. I did keep the 3.3 injectors, so that involved rebuilding the engine wiring subharness to let it plug into the main harness and have the connectors that fit the newer injectors and older distributor.
The R50 had different plumbing for the heater and coolant lines and the accessories are located differently. I simply used the mounting brackets from Frontier/Xterra VG33 engines and a steering pump from a Frontier/Xterra. My Alternator was from a 90's Quest/Villager and mounted up fine. I still don't have AC, after more than 13 years, but a compressor from an Xterra/Frontier should work fine, might have to get some hoses made to adapt. That let me use the flat multi ribbed belts the 33 uses without going through the fun and games of adapting the 3.0 harmonic balancer.
You can either extend the oil pressure switch wire or you can move the oil pressure switch from the oil filter adapter to the side of the block above the starter where the 3.0 has it, and the plug to the filter adapter.
Over all, it is a simple swap since the 3.0 and 3.3 have basically the same block and heads.