How can I lengthen the time text to speech pauses after it reaches a full stop ("period")
Use the following  after the word to tell your mac to take a break for 1000 milliseconds after that word:


  [[slnc 1000]]


"The silence command causes the synthesizer to generate silence for the specified number of milliseconds. You might want to insert extra silence between two sentences to allow listeners to fully absorb the meaning of the first one. Note that the precise timing of the silence will vary among synthesizers."

You can change the number of milliseconds.

Hopes this answers your question.
More information on https://developer.apple.com/library/mac/documentation/UserExperience/Conceptual/SpeechSynthesisProgrammingGuide/FineTuning/FineTuning.html#//apple_ref/doc/uid/TP40004365-CH5-SW11