This specific attack is not possible against SHA-1 (yet). The known collision weakness only allows generating two colliding pieces ahead of time with a hash va

This specific attack is not possible against SHA-1 (yet). The known collision weakness only allows generating two colliding pieces ahead of time with a hash value that you don't control. It does not give you ability to collide with a pre-existing file/pre-existing hash, so you can't attack any existing SHA-1-verified file. You can only attack files that you've specifically crafted/tricked to contain your colliding block.