Remove engine mounts from L Series X Members and weld into 1200 member to fit the engine low and far back as possible ?
I used the saddles from the Stanza x-member because it was just the easiest way to get suitable brackets in there for the L series mounts. To get the engine as low and rearward as possile requires mods to the mounting brackets on the engine and careful positioning of the saddles on the x-member.

Secondaly... can the same be done for the gearbox member ? or will i need to fabricate one myself ?
I have removed the gearbox mounting bracket on the drivers side of the tunnel for clearance, and so I could fit the reverse light switch. I have a manual tunnel which is smaller than the auto tunnel. You have to fabricate a gearbox x-member with different body attachment points if you have a manual tunnel. You may be able to use the factory mounts on the body if you have an auto tunnel.

Which gearbox would go in the easist ? no point doing things the hard way :)
Any of the Short boxes. Stanza 5spd or Stumpy Skyline boxes are definitely an easier fit than the long boxes, but the long boxes can be used as well.

How do the inlet manifold clear the strut towers ? close fit ?
http://datsun1200.com/modules/myalbum/photo.php?lid=1143

Custom tailshaft that mates up from the gb to diff ?
yes? You'll be using a larger splined L series gearbox output shaft, which is also further back in the body, and will have to mate to whatever diff you use that will also have a longer head than the standard one. You'll either have to modify some factoy tailshaft at both ends and have it rebalanced, so you may as well just have a new one made.

And do you have a picture of this Drag Link ?
http://datsun1200.com/modules/myalbum/photo.php?lid=1152
drag link is the steering link that connects both side of the car, running behind the x-member on 1200s. As you turn the wheel, this bar rises due to the geomtry of the pitman and idler arms. Mine clears the sump by only a few mm at full lock