Say a known SHA1 hash was calculated by concatenating several chunks of data and that the order in which the chunks were concatenated is unknown. The straight forward way to find the order of the chunks that gives the known hash would be to calculate an SHA1 hash for each possible ordering until the known hash is found.
Is it possible to speed this up by calculating an SHA1 hash separately for each chunk and then find the order of the chunks by only manipulating the hashes?
In short, No.
If you are using SHA-1, due to Avalanche Effect ,any tiny change in the plaintext (in your case, your chunks) would alter its corresponding SHA-1 significantly.
Say if you have 4 chunks : A B C and D, the SHA1 hash of A+B+C+D (concated) is supposed to be uncorrelated with the SHA1 hash for A, B, C and D computed as separately.
Since they are unrelated, you cannot draw any relationship between the concated chunk (A+B+C+D, B+C+A+D etc) and each individual chunk (A,B,C or D).
If you could identify any relationship in-between, the SHA1 hashing algorithm would be in trouble.