As far as transmission choice goes, it depends on how you will use the car. Do you plan to spend a lot of time flying across the open spaces Down Under, or will you be racing on a tight road course?
I just put in a 6-speed from Keisler that is rated at 650 ft-lbs, and I love it. It was pricey though, at around $5,000 USD. When I spoke with them, they were very willing to dicsuss rear end gear ratio, transmission strength requirements, installation tips, and even electronic speedometer wiring.
There are a number of companies building these high strength units. I don't know about the other companies, but Keisler needs some time to put it together. You may want to order ahead of time if you plan to ship it yourself.
Thanks Herb 68.
I really appreciate the input as I am going to need all the help I can get