1972 heater core

So I have a 72 mustang and I need to replace the heater core because it is leaking.  I am having a hard time getting the heater assembly out so that I can replace the heater core.  My manual is pretty vague and says there are 4 bolts that need to be removed but I can only find 2.  Pictures I have seen also indicate that there are only 2 bolts passing through the firewall.  I have been having a hard time finding resources on this and I was hoping someone here might be able to help me out with some information.  I would really rather not spend several hundred dollars to take it in and have it done if it is something simple I am missing.