OK, here's a small update on what I found looking at the trans this weekend. Primarily, this has to do with the speedometer output on the trans. Note, this is specific to the R154 AFAIK, and may or may not apply to the other "W-series" transmissions that Toyota used from this same trans family. There are two outputs on the trans, an analog for a mechanical speedometer, and an electronic for the VSS signal to the ECM (of the Toyota Supra). Since I hadn't really paid attention to this system before, as I was banking on just using the VSS output with an electronic speedo and as a feed to my EFI. Well, on a whim, I pulled the speedo pinion adapter (similar to good old Mopar stuff), and lo and behold, the threads appear to match the Mopar cable's big end: