I'm thinking I read a post from Dan Caron possibly some time back about mounting it by using wax paper or some other substitute under the pad and rolling this off the pad onto the firewall to ensure it goes on straight the first time. I could be wrong, but maybe a search using some keywords will reveal this information.
The dynamat option if you use a substrate to sandwich between firewall pad and firewall seems okay, as long as it is pretty thin and doesn't make things stand out too much. It's your car do what looks right to you, just use something made for the engine environment or you will be replacing/readhering the firewall pad in not too long.
1967 230SL