72-transit visa

My itenary is from usa to delhi, India---take a flight from delhi to Hongkong--then take the ferry to Guangzhou--spend 72 hours and then return to hongkong and take a flight from hongkong to usa. Can I do this with a transit 72 twov OR
Can I fly from delhi to gaungzhou, spend 72 hours, then take a flight from hongkong to usa---will I be eligible for the 72 hours free transit.
I am a US citizen