This isn't actually correct, is it? Assuming an average 6-letter word length, there are about 10^67 valid combinations that your iterate_all() can go through:
(((2^6) * 1000) ^ 12) * (94 ^ 5) = 3.5 * 10^67
Since this is much larger than the total hash space (2^160 = 10^48), you can hope you'll get a collision after the number of tries equal to half of the hashspace (10^48 / 2). That means you'll have to generate 10^47 hashes. At 5 minutes of runtime, you'll have to generate 10^45 hashes per second.
Bottom line: if you can run this on 200,000 cores, you'll have to generate 10^40 hashes per second per core, and that seems impossible to me.
I think it unlikely you have a compute cluster that can generate 3.3 million, billion, billion, billion SHA1 hashes per second (edit: and that assumes not adding the random 5 characters to the end). The actual contest will have a word list of a thousand words.
Also, (a) you're allowed to permute case of letters, exploding the search space even more, which is handy because (b) I presume the target sha1 will come from words not on the word list, making an exact match unlikely (hence score based on hamming distance).