I did 13B rotary turbo into a 116 transaxle type Alfetta GTV and had some vibration issues I diagnosed as being related to the engine speed driveshaft which was mounted to a carrier on the crank. The driveshaft was very similar to yours.
I'm a little hazy now on the details but I spigoted the tailshaft to the crank via an extension on the tailshaft's BMW/V6 Alfa Guibo flange using a teflon lined spherical bearing housed in the crank adaptor.
I worked out where the thing wanted to naturally pivot to allow for engine mount flex. This worked out a lot better compared to the standard locator.