You've gotten lots of good advice. Just to perhaps summarize and add my $.02 ....

* You can't do as we do in the Midwest. You have no comprehension of what the heat and dryness are like and high humidity high temp does not really acclimate you for the dessicating heat there. It takes about 3 weeks for your "thirst-o-stat" to adjust. Until then, when you start feeling thirsty it means that you are already dehydrated and when you feel like you've drunk enough, you really haven't. This means several things:
-- Adapt your schedule. Moonlight hikes are stunning in GC. (You can go to the naval observatory website to get the sun and moon rise/set tables for when you will be there. I always check that as part of my GC preparation). Get up early and hide in the shade (or in the creek) in the afternoon.
-- Don't go by feeling. You won't feel yourself becoming dehydrated. Monitor your ins and outs and urine color. If you haven't urinated in 3 hours, you are dehydrated. As they say, "pee pale". Dark gold also means dehydration. When you are dehydrated stay in the shade and drink until you are back to normal. In the summer in the GC you have only two potential dehydration states: A)Becoming dehydrated B)Already dehydrated. Active intervention is the only thing that will keep you in stage "A".

* Be aware of the impact of altitude: Wherever you are in Indiana is about 1500 feet underground at the Colorado River. The rims are a mile higher.
-- The sun is brighter and hotter than in the Midwest and there is a bit more UV in the mix, as well. Protect yourself from it.
-- Because of the altitude, it is also dryer -- even when it's not hot.
-- Expect somewhat of a performance/endurance hit. I don't notice it much except when walking out.

*Further about shade. Whenever you have a chance, be in it rather than in the sun. Good advice has already been given. One product to help is the GoLite Chrome Dome which is a reflectorized umbrella. Next time I go I'll figure a way to attach it to my pack so I can still use hiking poles.

*You can go down the S. Kaibab and I would recommend it for its unique stunning views. It's the only main trail into the canyon that goes out on a "peninsula" of rock. All others are in a "bay". Just make sure that you start before sunup. I'd take no less than 4 liters of water. Also, don't let gravity allow you to just pound down the trail -- take your time. The repetetive motion syndrome that you get there is known as the "Kaibab Shuffle". Be sure to use hiking poles. Going UP the S. Kaibab in the daytime has some potential for fatality.

*Use an electrolyte additive to your water -- diluted up to 50% from mixing directions.

*Having tried various materials, I have found my long sleeve micro mesh Caplilene to be the best keeping me cool and reducing sun. I'd carry an extra cotton T to soak. Also have a bandanna or something to have around your head and your neck and keep them soaked during daytime hiking. Light, loose long pants may even be appropriate. I like the zip-off approach. The shade principle applies -- the more sun you can keep off your body the better. When you are hiking, your legs are vertical and get less direct sun so shorts can be OK if you have been in the sun for several weeks prior.

*It's possible to go and have a great time. Many people do. However, I would never willingly do the inner canyon in July.

Have a great trip and give us a report afterward.

The issue with Clear Creek is that from Bright Angel/Phantom it is an uphill and then long hike completely exposed to the sun most of the way with no water. Again, to get there, you would have to adapt your schedule and leave maybe 2 or 3 a.m. at the latest. When the sun comes up in the morning it will be spectacular but will hit you in the face until you turn past Demaray point.

Plan on doing 1/2 to 2/3 the mileage your are comfortable in the Midwest.