The RB20 sump is a minor mod to fit on the RB26 to clear the oil pick up, the RB20 wiring is easy to add the few wires and connectors needed and the GTR ecu does plug in the same.
No RB20 heads do not fit Rb25-26-30 bottom ends, the studs are in the same place but not much else so this story is misleading as all the water passages need to be welded up and re drilled plus other mods and the tiny valves make it a waste of time.