This happens cause the mod is setting different id for the exactly same blocks in different worlds. And when im trying to copy an item with blocks added by this mod into another world, those blocks transform in a very ramdomly way (e.g. iron half-slab became a water source block in a new world). At the same time copied blocks loose all their TAGs when they are being loaded in minecraft.
Can you give any solution for this? Maybe to copy some files from one save to another in order to get proper IDs?

FML/Forge handles block Ids internally for 1.7.10. It might be better to ask in their forums.