GMH did do the bellhousings on 6cyl and Holden V8, and also the tail housings for 6cyl. I thought given the volume used in HD through to HT that they may have built the boxes here but as Dr Terry says possibly not. One of the reasons I thought they may be made here is there was always issues with the runout of a vehicle Series with predicted demand for ordered cars with imported driveline (vs stock order build cars already scheduled). We saw it in one way where lots of leftover imported engines/boxes were left - the two most notable ones being 307 auto engines and boxes where they had to fit them to HT to use them, and HQ where lots of 350 auto and steer sets were left, and they made a bulk run of XW8's right at the death of HQ to use up the mechanicals. Then in the other direction where imported parts were scarce towards the end of a Series - HG was an example of this. They finished GTS350 in December 1970, and restricted imported gearboxes that you could order from the end of March 1971: M20 stick shift was finished, M20/M11 (console shift) was very limited outside of GTS, M21/M11 was also very limited outside of V8 GTS and L31 manual was very limited altogether. N40 power steering was finished on 6cyl by that time too. Yet I have never heard of restrictions on ordering any automatic HT near the end despite Powerglide ending other than for HG GTS350. It is possible they did use them all up though as Dr Terry has told me a few times that some late HT were fitted with Trimatic - this may have been as they ran out of Powerglide rather than planned use of Trimatic?
Noting this really only affected ordered cars (as in Dealer, Retail or Fleet orders), the Stock Order stuff GMH knew well in advance what they planned to make and would have ordered stuff well in advance for them.