Heater Problems

[76/85 Truck]

This winter has been a cold one in Montana... and all winter I've dealt with a wimpy heater. At the end of fall, my blower motor went out, so I replaced that, but now it seems like I have no heat. When I put the switch on "low" it gets fairly hot by the floor, but when it is on high it basically blows cold air.

My first thought was that the blower motor is blowing the air past the heater core too fast, and as a result the air doesn't have time to heat up. What should I do? Should I replace the heater core with a new one and see if that will put more heat through?

Thanks for reading! Another thought was the core could be plugged or resisting flow somehow. Thanks in advance!