Heater or valve leak????? How to take it out????

I have a leak in my cooling system. When I kick it into passing gear and run it up in the rpm's, it squirts coolant out the heater duct onto the passenger's side foot well. I never leaks unless I hammer it. I smelled the coolant and could not find the leak. I finally saw it 'glisten' on the rubber floor mat. OK, now I have found the leak. Where is it? How much work is there to take the heater core out of the Dart? ('68 Dodge Dart 2-door)

I see the cover over the heater core. I have taken off the 'snaps'. How is the heater bolted to the body? I do not have the proper manual so I can use you guys expertise. Who has done this? Could it be the heat valve? Need a step by step guide to remove the heater core. Thanks!