Updated: Mar 31, 2020
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.