Decided to go ahead and insulate the main trunk of the duct work. My thinking here is, since I've saved myself so much headroom, I'm not going to mind losing a foot along the wall. To me, it's kind of a trade off. The furnace is noisy, and I'm going to assume the duct might be a little noisy as well.
I want to insulate my walls, and for that I need to be off the cinder block wall. Now maybe I don't have to be quite this far off the wall, but I do need to be off the wall. With all that said, if I wrap my frame with plastic sheathing and stay off the block, then my batt insulation shouldn't absorb moisture and shouldn't grow mold.
This should allow me to cut a few dollars off my build and leave plenty of gap to fix any issues I might run into down the road with relative ease (since I'll still have a gap to work with). Add in the fact that air is an insulator itself, I should be able to get by with less r value in the insulation (even though I'll still want sound dampening properties). It's starting to come together, even if I am not where I wanted to be at this point.