Replace heater core

I just bought this truck from a friend and the heater does not work. I purchased the heater core and my husband is having a hard time removing the existing heater core from this vehicle. He has never had trouble with our vehicles in the past so I believe there must be something he is not seeing. Any help at all would be appreciated. A walkthrough, with or without a diagram would be amazing. Thanks in advance for any help you can give.