Given the schedule changes, does that mean you are now on the 7:50 PM flight instead of the 4:50 PM flight? If there are free flight changes, can you change to the last flight of the night to buffer a little (assuming no fare difference)? 1 hr 15 min seems awfully tight with a 45 min baggage cutoff (on a good day, say the plane lands on time, it'll take you 10-15 min to get to baggage claim, wait 15-30 min for bags to arrive). Not familiar with MIA and customs/etc but 3 hours seems like a safer cushion.