Rebuilding heater box, replace core?

Quick illustration of the problem installing some of the replacement heater cores without damaging the heater box.
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The older replacement on the right is the one that fits.
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With just a little clocking during install the older replacement goes in and seats like this.
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Recoring cost was about $140.
I ran into this same problem. The new unit interferes and started to crack the box. I got a quote to record my old for $240. I thought that was very high but I don’t have much choice at the point.