The debconf question `partman-partitioning/default_label` should set the partition table type.
You also need to set the boolean question `partman-partitioning/confirm_write_new_label` to `true` or partman will not overrite an existing partition table.
So you should put in your pressed file :
d-i partman-partitioning/default_label select msdos
d-i partman-partitioning/confirm_write_new_label boolean true