ECCS is not a separate component, it's just what Nissan called the whole engine control system (computer, injectors, sensors, the lot). The "ECCS control module" is just the ECM. There's an ECCS relay, controlled by the ECM, which sends power to the injectors, the cam position sensor, the MAF, and one of the evap valves. That diagram's on EC-11 of the '97 manual. I don't know what model/year your donor is, but the '02 Xterra diagram looks quite similar, except that they renamed the ECCS relay as an ECM relay, and it doesn't control the injectors anymore.
IMO the deciding factor is gonna be how much stupid stuff like that Nissan changed between the two computers. Compare the pinouts in the service manuals. Hopefully they're close enough that you can get away with a little repinning. Maybe steal a few pins from the donor connector to get that boost solenoid hooked up. I remember My1Path doing something similar to put an M30 ECU in his WD21, but I don't think I've heard of anyone doing it on an R50, so you may be the first pig through this chute.
If they're stupid different, or the connector itself is different, or you're planning to hot rod the engine past the donor's original power, then the Megasquirt might be the way to go.
My personal favorite is Automatic Speed Control Device, aka cruise control.