Just to answer my own question in case someone else runs into the same issue:
It's a 13mm bolt for the bracket, and the best way to reach it is with a flex-head ratcheting wrench. A long-handle one would be preferable, but I was able to use a regular Gearwrench flex-head 13mm ratcheting wrench.
I had to modify the alternator bracket further. Since the bracket attached behind the plastic crossover instead on the aluminum crossover the bracket pushed the alternator forward too much and I feared it would stress the belt too much so I had to add two more holes. It looks horrible so I ordered a new one for 02-04 GT.