HELP: How to change heater core in 84 GMC 3500- no AC

[2badgn]

How difficult is it to change the heater core? I am not that great in mechanical ability with the exception of changing oil, door panels and detailing. I have spent alot of money on this truck and it would be nice if I could change out this heater core myself. The heater core is only about $40.
It was 20 degress in NY yesterday and the heat was minimal on high fan.
Radiator is full, heater hoses were replaced about 5 years ago.

How do I make sure that the problem is the heater core? Truck is 26 years old & it has never been changed. Where is the control valve located?

Is the heater core the box behind the glove compartment?

I have no problems using wrenches & screwdrivers but I struggle w/special clips. Can somebody tell me exactly what is required to remove it.

Thanks
Frank