IMHO heat is a matter of acclimatisation, just like altitude. The more often and longer you ride in the heat the better able you will be able to cope with it. Sadly that's not much help for a 600 just coming up. Preparing for the Mille Miglia in 2010 (Italy, August, bound to be hot), I tried to organise as many of my training rides as possible to be in the heat, and involving as many hills as possible, as those are the crunch points. Aided by a hot summer I was reasonably well prepared for the actual event.
There's a few other tips that might have short term benefit.
Take your cycling shoes off at every control and massage the feet. That should reduce sweating and tension that can cause the contact points in the feet to become very painful. Ditto the helmet. If it's really hot (as it was one day on the Mille Miglia, I soaked my helmet and put it on my head, which gave some short term relief but not sure about the longer term benefits.
Part of coping with heat is mental just like coping with a persistent headwind. You can't do anything about it but if you let it get to you it will be twice as hard.
If you can cope with electrolyte drinks then make sure you drink these as much as water or other liquids. I find that they upset my stomach, so I try to make sure I add salt to meals and have salty snacks (salt & vinegar crips, pork pies, and ham and cheese toasties all help)
Always stop in the shade, especially if you can find a bit of breeze. Avoid sitting in the sun at controls.
I considered trying to sleep for a bit during the day and riding longer at night but that just doesn't work for me.
There is a positive. With the current night temperatures being so high, you can wear less clothes at night and your legs are likely to keep warm, which should mean you don't slow down as much at night as you otherwise would.
Reapply sunscreen on the second morning. You'll probably sweat or rub off the previous day's application, even if you don't have a control with a shower.
Enjoy. It's probably better to be a bit hot for a few hours than shivering cold for a lot more.