Scrap the idea of the supra tranny.
If you really want a six speed in your old BMW, here's what I would
do. In fact, I almost really did do it.
For a short period ('94-'95 or so), the E34 Euro M5 had a six-speed
The engine in the car was a 3.8 liter S38 24 valve 340 HP monster.
Additionally, it was essentially a variant of the M30 engine that
in the 1968 2500. So, believe it or not, you can bolt the transmission
a 1995 E34 M5 to pretty much any BMW M30 variant engine, including
whatever you have.
I very nearly took the plunge in order to have a factory six speed
matched up to the '93 M5 egine in my '75 3.0Si. I sourced one out of
Munich Legends in Sussex, England. It would have cost about 6K to buy
the tranny new.
I ended up selecting the Metric Mechanic Sport/Overdrive option, based
upon a rebuilt
Getrag 265. It was cheaper, and I could actually use all the gears.
Another option is to find a "crash box" racing six-speed out of
E30 M3. These things would likely require a rebuild, but they, along
E30 M3 trannies, use the same bell housing as the Getrag 265, so they'll
up to an M30 Big Six engine.