2 boxes in my dart convertible. I did inside doors, floors firewall, inner rear quarters, basically everywhere i could. No problems melting. Im in az, and been out in 110F with the asphault temps likely in the 150s. AC kept the car reasonable comfortable. As far as sound deadener quality, worth the weight. Mine is a cruiser so i wanted comfort over power. Ive got heat shields on the mufflers and so far no melting of the insualtion. Stuff is easy to work with, and makes a world of difference.
I wouldnt definately do it again.