Mark, I always love your work! Great job on this. Too bad about your session horns trouble, those would have been nice to hear in this.
During the jam section, I found that there were some dead spots between phrases that I don't recall in the original. Maybe not dead spots, but rather, the "room" is missing. If anything, Chicago's mixes were always pretty full. I'm not sure if a different reverb would make it better, but I'd suggest looking into that. Lastly, there were a few (very few) places where the bass sounded too tight and mechanical, I'd loosen the timing a touch in those spots.
Oh, and more cowbell (and a tambourine!) :)
Regards,
Dan