And like the link between I-275 and FL 56, I have to ask why a southbound c/d road isn't being added to the project.
That is because it's in an earlier stage of design. I inadvertently came across some conceptual plans one day a couple of weeks ago at work (someone had printed it out on our copy machine), so I took a look at what was printed. Digging a bit further there are a couple of proposals, both of which involving a slight shift of I-275 south to accommodate the c/d roadway and the ramps to both I-75 and I-275.