I think there's an ongoing thread on www.clublexus.com that someone is trying to convert it using the Nissan 350Z 6 speed box, but other than that, I didn't hear of anyone that had done it. I don't have the link, but you can check under forums and SC430.
Interesting choice or trans, you would think W or R would make more sense. I wonder more about the ECU, no doubt they only get more integrated with more modern cars.
I've been looking around for a manual setup for my LS400, the mid 90's supra seems to be the best bet for me as of now, i imagine it would work for your setup aswell