hi dude
they're U-brakes not calipers.
when you say stack height do you mean the distance fromt he chain/seat stay to the front of the brake?
I'm guessing you have a chainstay mount of AD brakes and your chain might hit the brake as you hafve a small sprocket on your crank?
I have a Hombre on my ride, I'll measure it tomorrow for you, but would thing the hombre/ad 990 are pretty similar, you'll need a trendy and more expensive CNC'd brake like a fly I would think to get the profile down.
how ever i could be completely wrong.