The location of the crack also makes a difference. Also a "marginal" crack can open up wide with no warning. Any crack in a heat exchanger and the heat exchanger must be replaced, or a new furnace put in. As others have stated find out why the one you have now cracked. Could be an airflow issue from inadequate ductwork causing to great of a temperature rise. Airflow issues must be corrected before a new furnace is put in, or you will end up with a cracked heat exchanger on your new furnace.