Well, I ended up finding it. It had landed on top of the intake manifold of a project car in the garage. Didn't even think to look there as I could've sworn I heard it bounce along concrete and thus figured it was on ground level.
So with my joy of finding the c clip and getting the clutch stuff back onto the gear box, I now have a separate problem.
The thrust washer on the clutch "bottoms out" onto the c clip area of the brake cam before the gear box is flush with the mounting brackets. I'm making sure that the brake cam isn't interfering/hitting the spring in the clutch. I know the thrust washer/lower c clip are on the proper place on the shaft. What gives?