When I did mine it didn't tie into any heater core lines.
It simply went from the block on the back to a nipple in the middle of the cyl head on the drivers side. I actually had to pull the temp sender out and relocated it to the plug almost at the back of the head so I could redirect the water to the middle of the head. Interestingly enough my warm-up time has been increased by a few minutes but I don't know if that's just from the temp sender being moved or if it actually takes longer to reach temp now with the different water flow?