AAndrews said

Ok, so here's something to check.

Did you notice that there are two metal plates involved? One attaches to the heater box, the other is mounted on the engine compartment side , goes over the tubes and then is attached to the firewall with a single screw.

As well, the plate that attaches to the heater box is the one that has the holes offset. Look at the pic I sent to see how it is oriented before you attach it to the Heaterbox.

If you are going to repair the Heaterbox holes ( they look like they need to be redone) use fiberglass to do it.

Lastly, that heater core doesn't look correct. See if you can find a pic under the cooling section on this site to compare..maybe someone reading this can help...

If it is not exiting the Heaterbox correctly maybe that's why it is breaking up the box when it is tightened down.

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