The car originally had one mono speaker in the middle of the dash. No other speakers in the front.

If you haven't already, I'd recommend also asking this question on a BBS or newsgroup that is focused purely on Mustangs.

Yes, we empeggers are pretty knowledgeable, but your question is more of a "customizing a classic car for an aftermarket stereo" question as opposed to a "empeg installation" question. By asking in a 'stang group, you'll be able to get many more specific examples of things that other 'stang owners have done.