R200 (s15) Diff Gear Set Ratios

[alexauto]

Hi guys (and girls),
I am based in Australia and I'm building a new rally car which is using an S15 rear end. I'm having trouble finding a suitable ratio for the R200 diff - what I need is a 4.9:1 or 5.1:1, and although I was told they were available, the two sets I've now got from the U.S. have both been wrong.
I know that model names vary from country to country depending on how they were marketed, but if anyone could help me with a lead on where to get gear sets, or other Nissans that would be interchangeable, I would be really grateful.

Thanking you,
Alex K