I too replaced my heater core without cutting the new heater core inlet/outlet tubes. I did cut the tubes on the old heater core just to make it easier to remove but the new heater core had adjustable tubes and it was a bit harder to put in but it can be done. I got my heater core with adjustable tubes from NAPA.
If you have a cracked dash board this would be the time to change it too.
If you have a cracked dash board this would be the time to change it too.