Sorry Jeff and Carter. This is your race car thread, so if you don't want this all here perhaps it can be moved.
Summary:
i have a 4:11 Quaife R180 rear end in my 510 vintage race car.
I am looking to also make a 4:38 for the same car, as Jeff noted above, to have both as needed for different tracks.
I also have another 4.38 nIssan diff that I started putting a Subaru STi R180 CLSD in. That was going to be for my other street 510 with a 2.2 and 1983 MY 280ZX trans.
Before I started all this I also acquired an STi R180 CLSD with 3.90 ratio from a 2005. Not sure of any use for that one yet, it is an alternative for the above car if the 4.38 doesn't fit the needs of the street car.
There are Subaru R160 diffs available in 3.9 also, but most of them (at least the newer versions) are VLSD.
I Know that guys use the 3.36 or 3.54 with V6 engine swaps, so maybe yours with a 6 speed ( depending on ratios) would be ok with a 3.54. The later subaru STi diffs from 2007 are Torsen. Most are 3.54 but i guess there are random years in there that also use a 3.9 again.