The t-stat housing bolts are 13mm. I usually remove the t-body and use a box end wrench just to make it easier. Don't foget a new t-body gasket.
Here is a grand am 3400 write-up courtousy of the grand am forums. Its almost identical to the 3100 for t-stat replacement.
http://www.automotiveforums.com/vbul...d.php?t=919386