1968 Firebird convertible -- has anyone used Dynamat, Second Skin or Hushmat for sound deadening products on floor board under carpet, trunk and/or door panels? What is your suggestion or experience with such products? Is such a product worth using on convertibles?

There is your key question. If the top is down 95% of the time, it won't do a thing. It will only help slightly with the top up because so much noise comes through the top and the well.
But it does help with engine and road heat cooking your legs and toes!

I agree with Craig... the only thing it will do is reduce the heat in a convertible, which it does well.
We used HVAC duct wrapping from home depot.
It has an adhesive back with about 1/4" foam batting and aluminum foil cover which applies just like the other sound deadening products. Works well with heat deflection.
We did use it in the doors but in hindsight would not, although they sound pretty solid closing.

The home depot stuff was much cheaper and less than $75 to do the entire car.

I'd certainly use it on the floor boards and center console. No on the doors and trunk.
Just my $0.02

make sure to do the rear wheelhouses, trunk divider, and by the rear window regulators. It stops a lot of road/exhaust noise and echoing from the trunk from the not so dead bodies. lol.