If you swap a compression-wise like-for-like block (ie. a B23F for a B230F) and reuse all the ancilaries (manifolds, injectors, sensors, etc) of the old engine, in other words swap the bare longblock, you can keep the old ecu and wiring.

If you include the manifolds, sensors, etc in the swap, you'll also be swapping the AMM, maybe/probably (parts of) the wiring harness, the ECU, etc, as you'd essentially be downgrading to an earlier version of the FI system.

Flex plate will transfer.

Yes, you can swap the shortblock and reuse the head &c. Depending on which head castings you have (531, 530, 389?) that might actually be beneficial. The crankshaft mount holes never changed. The reason for different flywheels and flexplates has to do with holes/slots for the ignition sensor, and on manuals the recessed clutch of the M47-transmissioned cars.

Changing from turbo to non-turbo (higher compression) block you'll need to change the ECU over. Going from B230FT to B23 you'll also need to check the partnumbers for injectors and sensors of a B230F (keeping the system modern) or change the whole lot over to the earlier system if you want to use the B23 stuff. Personally I'd look for an ECU from a similar year/version B230F to keep the wiring/sensors/AMM the same.

Bram