You should change the function that corrects the column number. So replace lines 49 - 56 by these lines:
def correctCol(col):
# starting at AA
return col + 26
This will yield a column number that will start at AA (=27).
The error that make your rows start at 0 (probably due to the extent defined) can be easiest (quick and dirty) corrected by simply adding 1 to the row number at line 25:
rownum = getRowNumber(ext.YMin, ymax, height) + 1
The alternative would be to check the extent defined and adjust accordingly.