Beans was running one in their pulling truck, and I'm pretty sure they still are. From what I heard it was holding together just fine at 750hp with totally stock internals, and they eventually started having problems with the syncros.
It won't be a fast drag truck, but the trans will hold some serious power with the right clutch.
I have an idea... Sell me your ZF6 and buy yourself an auto... LOL
My buddy has a ZF6 behind his 600hp 7.3L and hasn't had a single problem with it aside from the single disk clutch getting fried... A SB dual disk has held up just fine here lately though.