Possible Clues:

• "You can count on that!," and a hint to the starts of 16-, 25-, and 43-Across
• "You can count on that!" ... or what you may do with a 17-, 27- or 42-Across?
• 'Believe you me!' ... or what you can do with the start of 18-, 24-, 53- or 63-Across?
• Assume with confidence