I went the cheap way on my 70 Challenger, though I was focused on noise (Michigan doesn't have a BIG problem with hot weather).
I went to my closest "Pick and Pull" junkyard and got the under-carpet insulation out of a late 80's "AS" model Chrysler T&C minivan. $20 for the whole floor area, enough material to do two cars. Nice thick insulation with a sound deadening mastic bonded to it. The later 90's insulation is not as good, I suspect more of the task was accomplished by pieces bonded integrally to the floor.
Besides putting it on the floors, and trunk partition, I used 3M upholstery adhesive to glue it to the back of those nasty hard plastic door panels. You need to trial fit it before you glue, as some areas near the armrest are critical to door panel fit and attachment locations.
Seems to have civilized my car some.
Mark