GM did make a 4-sp version of the Trimatic but it wasn't fitted to Holdens sold in Australia. It was used in a few export versions.

It was however seen in many BMWs sold here during the 90s.

The GM name for it was the 4L30 (there was also a 4L30E electronic control type).

My guess is that you would need to make a new bell housing using the front half of a Holden unit
& the rear half of a BMW unit.

Dr Terry