The only person that I've ever seen do this is TwinTurbo, who doesn't come to CF very often any more. As I recall the bell housing is different, which required modification, you needed a hydraulic clutch ( no big deal) but probably the biggest issue was that the shift lever was noticeably further back. Iirc, he moved the motor forward to mitigate this.
I also think that you'd have to significantly modify the tunnel on a C3. The ZF is a bit of a monster.