You also need to consider if the CA18 flywheel will fit inside the CA20 bellhousing.

The bellhousing flywheel/clutch thing is something else to consider, on top of 'will it bolt up'.

If the CA18det flywheel and clutch fits in the ca20 box, that's great. Otherwise you might need to use a CA18de flywheel and clutch (which is smaller). Maybe even a FWD flywheel and clutch, which is smaller again.

With all this chopping and changing, you might need a different starter motor.
I have a feeling that someone else on here is running a CA20 box in their CA18det Ute...