Alan, do you have the carpet installled? If not, that will make a huge difference. And did you use the firewall insulation pad or leave it out for weight savings? Same deal with it, it makes a big difference.
The majority of the heat will come from the firewall and roll out from under the dash. Headers not only heat the front of the floorboards, but the firewall too. The floor fresh air vents will blow warm air at first, since that's where they get their air, but as air is passed through them, they typically cool down and really help. Ross makes a good point about the front air dam. I hadn't thought about it before, but that probably does help draw air out of the engine compartment and keep it cooler.
I don't have an issue with heat in my car, but I also put Dynamat all over the floor before installing the carpet. That stuff really helps with header heat in the front floorpan area and also dampens road noise. Drawback is it's heavy. I also have the firewall insulation pad, and although it helps, I will still get heat that rolls from under the dash. I wish I had put Dynamat there also, but it's not bad enough to tear it apart and put it in. Even on the hottest days, it'll get a little warm in the car, but it's not so bad that it makes it uncomfortable, at least to me. Keep the floor vents open at all times.
Edit: Oh, and make sure you seal up ANY holes on the firewall. Any hole that isn't sealed will push through hot air like a house furnace.
