After pouring over the Kanji DD skyline wiring diags, and realising there is absolutely nothing similar with the ecu (other than the connector) or the Re4 transmission controls, and not being sure the 'can' system would even communicate, we came to the conclusion the det ecu won't work with the 4 speed, and the DD ecu won't drive standard injectors, (or Emanage) without extensive rewiring/modification. It seems to be a big can of worms, around 200 wires to reroute and no guarantees it will work.
I think the turbo would have fit, but required custom cooler piping, as the starter motor is 3 inches higher. Also tapping turbo feed and drain fittings into the RWD sump was needed, as the RE4 and RE5 bellhousings are different.
The engine mounts were 4 inches closer to the engine, meaning the turbo intake would need to be custom made.
The chassis was different, and the Stagea front subframe wouldn't bolt up. This would mean major chassis mods, cutting parts off the Stagea wreck and swapping them onto the Skyline to remedy. This would make the car defectable. Possible for a track car, but obviously not a road car.
Sad to say it, but it is just cheaper and easier to buy a Stagea Mark... You know you want one...