Lots of good ways. First start at the source. Wrap you headers and pipes back with Thermotec header wrap. This may shorten the life of the header, but do it anyway. Then start with some of the heat reflective materials that they offer. Depending on what's going on in the undercarrige you also can make simple metal heat shields that are very effective. These will do a huge amount to reduce heat. If still bad, attack from the top layer. Pull carpeting and make sure you have some insulated mat under the carpeting. You can get this from Thermo-Tec too I think. Other companies make the same sort of products (Dynamat), just familiar with them. Last thing, and I have not tried, but others seem to have good results are products like Lizardskin. It is a paint on product, but seems to have some good feed back when I last was looking into using it. One thing to also watch out for in the inside stick on materials is water collection which can lead to rust. Paint on doesn't have it as bad.

Last night my wife installed Dynamat Xtreme over the entire floor pan of our '67 Buick GS convertible in an effort to reduce heat and noise in the interior. The product is easy to install but I recommend that you wear gloves since the foil backing will give you cuts similar to paper cuts if you aren't careful. The material cost was about $210 and added 24 lbs to the weight of the car. I'll post pictures later and let you know if we notice a difference after we drive the car this weekend.

Well how much dynamat did you need to buy for the whole floor of your 65 mustang.