Ciao Lee11,
The free parking areas you mention do not have closing hours so you won't have problems with heading there late. I think the one you'll find most convenient is the one in Piazzale Michelangelo although I wouldn't leave the car there for over 24 hours. To leave overnight, I'd go for the one in Ponte a Greve lot since it is well connected with the tram into downtown Florence. Buses and the tram do run in the evenings. Check buses that take you downtown on the page with the info on the parking lots (Where to Park in Florence, Italy? Map & List of Parking in Florence, Free and Paid Parking Lots) and then go to ATAF net to check the schedules for each line. The tram operates from 5am to midnight with trains every 9-10 minutes in August (every 5-11 min in September, depending on time of day).
Hope this info is useful!!
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