I hate to ask the obvious but here goes. Are you sure the motor is turning the right direction? The fan blade is not on backwards? Im really not familiar with what you have so forgive me if I'm asking stupid questions. Your airflow just dont seem right. Does it pull air over the heater core or push air through?
Jeff

"Do not walk behind me, for I may not lead. Do not walk ahead of me, for I may not follow. Do not walk beside me either. Just leave me alone, you're starting to freak me out."

1957 GMC 150