Chunk 2: 64 bytes
merkle_root[3:0]
nonce
timestamp
target
Chunk 3: 64 bytes
digest from first sha pass
Their improvement requires that all data in Chunk 2 is identical except for the nonce. With 4 bytes, the birthday paradox means collisions can be found reasonable easily.
If hard forks are allowed, then moving more of the merkle root into the 2nd chunk would make things harder. The timestamp and target could be moved into chunk 1. This increases the merkle root to 12 bytes in the 2nd chunk. Finding collisions would be made much more difficult.
If ASIC limitations mean that the nonce must stay where it is, this would mean that the merkle root would be split into two pieces.