Engine and box as a pair. Engine wiring loom and ECU will extract in one piece, but there will be numerous cross connections to the car for various switched and unswitched power supplies, tacho drive, fuel pump control etc. Nothing insurmountable.
Z32 has radiator electric fan control and also has an input from an electronic speedo.
Check a Z32 wiring diagram for the details of the EFI harness plug F2 connection to main harness plug M64 - I think this will give a simple way to hook up all the external devices and power supplies. The Z31 has an equivalent plug interface between ECU loom and car just above the ECU location in the passenger footwell...
Custom propshaft, custom rear crossmember / mount. Front mounts may adapt from one or other engine (or a combination of both) as the blocks share a common heritage.
It's been done in a 260Z, so should be a piece of cake in a Z31!