Taking into account where you live I would be tempted NOT to underseal the cab/firewall.
I've never been a great fan of these textured finish underseals,preferring to just keep everything clean and smooth underneath. That's even where I live in a salt laden winter road situation.
I feel these underseals encourage build up of dirt,crud etc due to the texture. Personally I prefer to be thorough when cleaning,i.e. pressure clean under my cars regularly.
My own truck came from So-Cal and survived nearly 60 years without any underseal at all.
The other benefits claimed for the Dupli-color is sound deadening,this can be achieved internally in the cab.
That's just my $0.02.
