Ok I am no greaser but I have learnt quite alot from just reading on this site.

The 180b crossmember is pretty straight forward to fit to a 120y, But the B110 is narrower so it is a lot more work to fit the 180b crossmember. ( havent read of anyone doing it, but anything is possible)
The reason the 180b crossmember is used in the 120y is because it bends the opposite way the the 120y crossmember and this provides oil sump clearance for various engines.
If you get the 12A engine mount, the engine wont mount to the crossmember. It will mount infront of it on the chasis rails. The 12A mount was suggested as since it is to the front of the engine, there is now more room beside the engine for exhaust manifold/pipes etc.
The mounts and bracket to my understanding are like an engine plate. One plate bolts to the front of the engine and to the sides of this the engine mounts are attached. This is only on 12A's but you can attach this 12A timing cover to a 13B if wished.
The 6.0mm angle refered to when describing a gearbox mount would be 6.0mm thick angle steel. Not to provide correct height or angle, but to provide strength to the mount.

Hope I helped, and all this I found from reading www.datsun1200.com search results.

Melissa.