I built my soffits very similar to the way those I-beams are constructed - two 2x2s screwed into a strip of OSB. So they should work. Are all your soffit heights shallow enough that the I-beams will work? My soffits ranged in height, from 7" to almost a foot or so, depending on what I was covering.
I also found that the bottom of my floor joists weren't exactly level going across. So I cut the OSB an inch or so long, screwed the top 2x2 onto the OSB, temporarily screwed it to the bottom of the floor joists, then snapped a level line across the bottom of the OSB at the finished height. Then I pulled it down, cut on the chalk line, screwed the bottom 2x2 on, and did the final install. I'm not sure how much leeway you'll have with the I-beams for adjusting to level unless you shim some of the floor joists when you attach the beam.