You want to make sure you get on the earliest bus possible from Harare as the border into Mozambique shuts late afternoon and thats one border you don`t want to be stuck overnight.
The journey takes around 3 hrs by car to the Mozambique border and then you want to get to and across the the Malawi border, its a long way by car from Harare to Blantyre including borders its a dawn till dusk drive.
Onroute is Tete in Mozambique, a very depressing town and not worth spending any time unless your not in a hurry and want to see some bullet ridden buildings.