Are you having an issue with the player colliding with these parts and not allowing the player to move past? There is no way to get the chunks specifically via BP and alter them.
If you want to change the collision for the chunks that are blocking the player you can do this by selecting the DM in your scene and in the details panel you can alter the “Large Chunk Threshold” to make any chunk below that threshold not affect the players movement.
You can take a look at my simple tutorial covering this: [TUTORIALS] Photons Be Free: Mini-tutorials and other curiosities - Community & Industry Discussion - Epic Developer Community Forums
If you need something more specific can you please clarify and explain in more detail and I can see if I can help get you started.
Thanks!
Tim