Brute forcing a partial block is just as hard as bruteforcing the whole thing - you still need to create a second string which hashes to the same as the first one.
Simply trying all the values from 0x00 to 0xFF will (statistically) never result in 2 blocks with the same value since you are only bruteforcing 8 bits and the output of the hash is 512. The chance of two arbitrary blocks matching, regardless of length, is 1/2^512.
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Brute forcing a partial block is just as hard as bruteforcing the whole thing - you still need to create a second string which hashes to the same as the first one.
Simply trying all the values from 0x00 to 0xFF will (statistically) never result in 2 blocks with the same value since you are only bruteforcing 8 bits and the output of the hash is 512. The chance of two arbitrary blocks matching, regardless of length, is 1/2^512.