Howdy,
Brabham, try Nissan part no.38100-N3405 which is a 3.154 ring and pinion set listed for an R200 in the Datsun Competition parts list. I very much doubt they would be avalible new, but try US sites, as they were sold there new. Dont waste your time with Nissan Aust, the US is the answer, if you want one.
Also try www.locost7.info/gearcalc.php and dial in your numbers, It will tell how many RPM you will need to do what speed in what gear, It will be up to you to get the power to pull the numbers!!!
If you can get hold of a DVD called 'Big white Dyno', done at Lake Gardiner in '04 it will show you how hard it will be....
If you look at the DLRA results not mant cars do 200MPH (320kmh) goooood luck.