I did the exact opposite swap on a '72 1800E. It came with a BW35, with a shifter occupying the place the gear shift normally would. I don't know if the tunnels are different on an Auto vs. manual cars - you don't have a clearance problem going to the smaller manual transmission. It was a pretty straight forward swap going that direction - the proper mounts for the clutch pedal, crossmember, etc were all there.

It wasn't much fun driving with the BW35. It was slow off the mark. It was slow on the roll. It felt sluggish at highways speeds. And after about 3 or 4 months of exuberant bot not abusive use the shifts began to slur, and it started slipping at full throttle in first gear. Just as had happened in my previous 145E wagon with a BW35. After the swap the car was much more pleasing to drive.