Rebuilding heater box, replace core?

I went through hell with mine. My original one was not leaking, but since I was rebuilding my heater box - AC too - I attempted TWO different aftermarket versions (one number started with "CR") and ended up getting mine tested at a rad shop and put her back in!
If those tubes are off from where they should be, you will STRUGGLE. They can be heated and corrected, but why do that on a NEW part you just bought?