Heating the cab pickle! Help please

[nateshrek]

I have a 65 gmc, and I just replaced the heater core under the hood. The blower motor was replaced at some point (not by me) and seems to work fine, all three speeds. All the under dash hoses are connected with no clogs. After reading maybe people saying their new heaters were roasting them out, I figured mine would do the same. But after flipping the switch, it's more like a light to medium fart, not hurricane gussy. But seriously it barely blows enough to feel with your hand over the vents.
Any ideas as to what I can do to get more airflow into the cab?
One thing I may add, when I installed the blower motor with cage onto the unit that bolts to the firewall, the hole that is in the box, right overtop the top of the cage, was scrapping. Is that tolerance between the bottom of the box and the cage to close not pulling enough air?
Please anyone with ideas, let me know

Thanks guys.