Dynamat or Fatmat??

Ya'll really like to spend money: The most effective ways to Thermally Iinsulate is to install 'mass' & to tighten-up the 'envelope'. This means that you need thickness &/or density in your insulation. Incidentally, these are also the primary requirements for Sound Attenuateion.

The most effective (& economical) way for you to achieve both of these goals is:
1) MASS: Get some nice carpet padding (you might even find some for FREE) &
just put it under your carpet. If you want to get fancy & spray on some sound
deadener, undercoating, or other thick (& ROUGH textured) compund, go for it.
2) TIGHTEN UP: Tape Up/Seal all holes, openings, cracks, etc. Sound, especially,
will travel around any opening. Thus, any of your hard work will be negated if
sound has even the smallest way to infiltrate the space. This is especially
critical if you've spent a lot of dough on DynaMax, or Fat-Axx, or whatever.

3) RADIANT BARRIER:If you want to feel cool with that foil-look, then at least do it
inexpensively. A Radiant barrier is really not needed on a car's underside. The
main heat is mostly localized to the exhaust & engine, so those are the
only areas that may need it. Go get aluminum foil at Wal-Mart (or better yet,
Family Dollar). Then apply it w/ that Super77 or other spray adhesive. If you
want, you can try using Boards of Rigid Insulation w/ the Foil facing (abt $9 per
32sf/4'x8' sht). It will provide some themal mass & a radiant barrier, but will be
less effective w/ the acoustics. If you used the rigid board w/ carpet padding,
though, you'd get a pretty good assembly. It'll be thick, about an extra
1-1.75", but that isn't too bad in an A-Body.

FWIW, I design bldgs for a living (at least in better times). A large part of my career dealt w/ designing Broadcast Audio Booths/TVStudios,etc. So I have a bit of experience to bring to the game. Also, I did the Carpet Padding route in my '67 B/Cuda & it really makes a big difference. I do have the std leaks during rain, so the need to keep the car dry is important w/ the extra mass.

If you attack the actual LAWS of Thermodynamics & Acoustics, then you'll make a quieter & cooler interior. However, if you only look to the LAWS of Marketing, then you'll spend too much money of stuff that will provide little satisfaction.