Transmission. It is recommended that you use the 4-speed out of the B210 as it is the 60-series (heavy duty) to handle to torque of the A14

However, if your 1200 already has a 56A (heavy duty) I would use that instead. Because it's already there and bolts to the 1200 transmisson crossmember. Most 1973 1200s have this gearbox, but you can look on the side to verify

If you use the B210 transmission you need a simple adapter for the transmission crossmember

The flywheel/engine plate/throwout lever combination is critical. If you're using the engine+transmission out of the B210, no worries. But if you mix the A14 with the 1200 transmission it depends on which tranmission you have

My notes say the B110 pan may have problems on an A14

* It may work with some tightness in fitting the dipstick
or
* Definitely works: use pan from Datsun 210 (1979-1982) A12A/A14/A15. Use B110 or 210 oil pickup tube

early pan has oil dipstick going inside block rails
1974-up pan has oil dipstick going through hole in block rails

When you get to that point, can you take some comparison photos of the two oil pickup tubes (B210 tube vs B110/210 tube)?

I have a never been used 1200 crossmember modified to fit a14/15 motors that I'd part with but to be honest the flat plates of metal shown in the tech section works just fine.

The motor mounts on my race coupe are nothing more than flat pieces of metal with holes drilled in them. It's not pretty but it works, also note my motor is pushed back an extra inch so the holes are 2.5" apart and I've had no issues.

Tom