Heater Core replacement - '67 A Body

A couple points that may help. Remove the front passenger seat. Makes the job a LOT easier.
You may want to order the foam/seal kit from DMT(Detroit Muscle Technologies)...its not expensive.
When you get to the point of "splitting" the box/case where the heater core is, be VERY VERY careful as the clips that hold them together are extremely snug and you may crack/break the case.
When putting the case back together "relax" the springs/clips a bit so as not to put too much stress on the case.

Here is a link that will help
http://www.hotrod.com/cars/project-vehicles/mopp-0311-1967-plymouth-valiant-heater-box-assembly/