Q:

How does the pronunciation of “dass” compare to “das”?

I’m reading a book in which these two words often occur right next to each other as so: “… dass das …”  Is there a difference in their pronunciation?  If so, how can I make that clear?  Do I need to use a certain pronunciation to differentiate it from a phrase like “… das das …” for example or is the context sufficient for that purpose?

A:

There is no difference in pronunciation (not in Hochdeutsch at least), and you don't need to differentiate them, since grammar orders them. 
That is:

das dass  (✗)

is forbidden by grammar rules. In  particular, dass requires a punctuation sign, in fact:

das, dass
  das. Dass
  das; dass

are possible.

