If you must - here are few answers to your specific questions---
122 and early 140 series cars use what is called the "long tail" version B35. It does not use an external oil cooler - it is air cooled (has to have the cooling fins on the torque converter).
It's been a long time but I think the crossmember is the same M40-B35. M41 crossmember is different.
Again, been a long time but I think an M40 front shaft matches a "short tail" B35 and an M41 front shaft matches a "long tail".
You'll need a "6 bolt" flex plate if your motor is original. If the motor is a later "metric motor" ('74 &'75) you'll need an "8 bolt" flex plate. One from a 240 series will work.
The '69 had a column mounted shifter-- the PRNDL unit is cable operated and fits where an M40 car says VOLVO or an M41 car mounts the OD lamp on original clusters. To mount a later style floor shifter will require modifying the transmission tunnel- driveshaft hump.
As others have posted - the B35 is crude compared to the BW55 or AW70-71. It does not do partial throttle downshifts well, if at all. Given all the work involved, going the extra steps to fit a later transmission (AW70-71 with OD) would be well worth it. -- Dave