I think you're right on the USA only. Perhaps Australia too (there was an old listing for 2.88 diffs down under). I've reached out to someone in the States who's checked their parts people, but no dice so it looks like custom fitting is the best option.
Spoken to SimplyPerformance who have done the conversion before, and it's easier if you can provide a '40 diff and a 2.88 LSD core which I've got knocking around
watch this space!
On an aside, where's the best place to put up a project thread?
Also, hell yeah biturbos. Don't own one myself, but I've always loved the things. My dad's got a 228 and a 430 and they're brilliant cars. Always wanted a SWB early coupe with the later injected engine myself...