I work at Ford and have responsibility for sound insulators and carpet on several programs. When it comes to sound insulation, the method of control depends on the source and the frequency of the sound.
Low frequency sounds (suspension noise, engine noise, exhaust) respond well to mass. Dynamat, etc. Weight kills low frequency sounds.
If you want to address higher frequency sounds (wind noise, etc), the answer is a light fluffy absorber. A cotton shoddy (sheet of tightly packed fiber threads) is a good, cheap way to go.
Typically, a good sound package starts with a good dash insulator first and foremost. Get one with a soft face on the front (facing firewall) with a harder, heavy face on the back (facing the driver). This will give you basic sound insulation. To go one step further in the IP, take pieces of loose shoddy or other absorbtive material and place it anywhere there is room between the dash insulator and the IP, duct work, etc. You can staple it to the back of the dash insulator where you have room. For most benefit, don't compress the material when you install it (just like when installing the pink stuff in your house).
On the floor, the best bet is to add mass as most of the sound coming through will be low frequencies (suspension, engine, exhaust, etc) that propagate through the sheetmetal. Use dynamat, spray on, etc.
Make sure you have a complete door seal - the big plastic cover that seals the wet part of the door from the dry inner part is a must. I have replaced this piece with dynamat before with good results. Just make sure it covers every opening that isn't used by a moving part. Don't put any soft insulation outside this seal - in the 'wet' zone - but covering the back side of the door trim with it is a good idea.
Improving wind noise on an old Mustang is a challenge but you can help the situation by stuffing lightweight absorbtive material behind all the trim (pillars, door panels, quarter panels, under the package tray, etc).
There are some big passages between the trunk and passenger compartment that can use some stuffers - for ease of install, use the soft stuff. Seal off holes in the package tray before installing the trim.
Cover all unused holes in the body. They are 'hot spots' as far as noise intrusion is concerned.