Thanks for all of the quick input, guys. I've 6-2 and have size 13 feet, so the tight footwells and shorter length might be a problem for me on the 763, and there wasn't availability in Row 1 or any middle seats.
Just booked ICT-ORD-GRU on the 777-200. 9A and 9K, but might switch to 6K on the return leg (UA shows it available, but our company's travel agent didn't).