If your flight times are "reasonably" on time you should have no problem making it to the ship on time. Cabs are plentiful at MIA and the trip to the port is only about twenty minutes.
The problem occurs if there is a delay in your flights. You need to be onboard ninety minutes before sail time.
One thing to remember is that almost all cruise line air flights arrive on the same day. Your revised time gives you more time but it's still close.
I advocate arriving the day before the cruise but there are circumstances that make this impossible and you need to do whatever you reasonably can in order to make the cruise.
I also advise you to take out insurance, in the event things don't work out.
Cruisemates Community Leader/Moderator
Upcoming Cruise(s) Carnival Liberty: May 31, 2014. The Anniversary Getaway.
"There is a great difference between being well traveled and just having been to many places." ~Me
"Fear is the assassin of dreams." ~Me