Wow, thank you so much for all the information! Way more help than I was ever expecting, it's not hard to see theres a lot of genuine enthusiasts here! I reckon you guys have already saved me many hours of future frustration and a fair few dollars to boot.
That info about the TKO 600 is really good, had never even considered using that box before, it looks like it'd be pretty much perfect with a .64 cruising gear. Looks like they are readily available too, either importing from the USA or found
Mal Wood Automotive - Tremec in Australia