I am guessing that it needs to be a word (as in password),
This was part of the reasoning behind pairing them up as the values -3 -2 -1 0 1 2 3 4 will probably not be enough to replace with a letter (a word or sentence using only 8 different letters and being 44 characters long, not likely)
Another concept I was working on was starting somewhere in the alphabet and move forwards or backwards the number of letters indicated. The problem with this is that if you add up all the digits it is greater than 26.