Are you saying that the cast iron block has corrosion issues, but the aluminium cylinder head is OK? That's a new one on me.

The 1200 GX [with a capital X] engine has more compression, but my parts book says that the pistons are the same, so it must be the head that provides it.
The camshaft is diferent & i would not lose it if i were you, even if you don't use it.
The coolant suction pipe at the front of the block is diferent too as it has an extra fitting for the heating system in the inlet manifold.

These seem to be the only things that i can find that are diferent between the short block 1200 & 1200 GX engines. [1970 to '73]
The cylinder head, & just about everything that is attached to it is diferent, but you already knew that