Although I have never done a heater core replacement I understand that it is a real PITA. I also was having some leaking issues and did not want to dive into replacing the core right away. I went to the auto parts store and bought a double hose barb fitting to connect the two sizes of heater hose (one end is 5/8" and the other is 3/4"). This will allow you to bypass the core like you first thought to do.
