Chris,

There's always the AW7x conversion option. I have an AW71 from a '94 940 turbo in my 1969 Amazon, behind a stock 1974 B20. The conversion is a bit fiddly (and not inexpensive), especially since I was converting a manual tranny car. The BW35 cars have a wider tunnel so there's no tunnel surgery required, plus you can use the stalk mounted shifter and the throttle kickdown lever on the throttle shaft and the brake pedal is in a better place.
The .69:1 O/D ratio makes highway driving lovely and quiet with a commensurate boost in fuel mileage. There are only two areas that are less than ideal, one being that the stall speed of the 940 convertor is around 1600 behind the B20, which means the car is a bit sluggish off the line, plus there is no speedo drive on this vintage of AW71, which meant I had to rig up an electronic speedo off the driveshaft.
There's a lovely example of this conversion in an original BW35 122 up in the Everett, WA. area. I could probably find the contact info of the owner if you were interested.

Cheers,
--
Barry '67 122S "Betty" (Rest in parts), '69 122S "Veronica"