Yes it would. You'd also need the DME out of the donor M3, and would need to research to confirm whether your car is OBD I or OBD II. (On Board Diagnostics). If your car is OBD II, then you need to research OBD II to OBD I conversion.
Good luck, welcome to the 'Fest, now post pictures of your car or be banned from this forum
The 95 M3 engine is OBDI. The 99 323i is OBDII. You would probably need a new wiring harness as well as the M3 DME. The OBDI cars only had one O2 sensor post cat where as the OBDII have one pre cat sensor and post cat. Theirs a few other differences.
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