Wow, this was an old thread.
Powerful Build does NOT count your UAS damage as large. It allows you only to wield a weapon one size larger. UAS damage is a function of your size only, not what size weapon you could wield, so it doesn't help.
SUAS does one of 2 things. It either gives you UAS damage per the table, OR it gives you UAS damage equal to a monk 4 levels higher, if you have monk levels. So, your monk has levels, and thus would have the UAS damage of a medium sized 6th level monk, which, IIRC is 1d8.
Also, Reaping Mauler is terribad. I hope you don't have the Mountain Rage ACF from Races of Stone (the book goliaths are from). If you do, when you rage, you become large, your Clever Wrestling feat turns off, and you lose all of your Reaping Mauler abilities forever. And that's not even counting other enlarging effects like Polymorph.
I'd suggest looking at Complete Champion for the PRC Fist of the Forest. It will give you 2 size bumps on your UAS damage, along with Con to AC which will stack with your monks Wiz to AC. Or take some levels in Warblade and pick up the Crushing Roots of the Mountain stance.