The 4.10:1 was the torsen diff... It was only available on the JZA70 model (possibly the GA70 as well) and only an option at that. It does not do well in high torque applications though... It is VERY unpopular in the Supra community as far as mid to high end builders are concerned.
I believe Kaaz made a unit which was compatible with the carrier which was supposed to hold up better but I don't think it was great either... All of this is rather sad as I really prefer torsen for street driven vehicles.
With regard to what options are available I'd try emailing or calling the people at MarlinCrawler.com.
Honestly, given that you're already digging in for a custom setup there are a number of options from other manufacturers I'd look at versus keeping the MKIII's rear end. (I'm sure you've already got this in mind though.)
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