[NOVAGOAT]

I am replacing the heater core on my 69 ram air III GTO, and it's been challenging. I ordered three heater cores from different suppliers, including Ames, and none of them fit. The tubes were not far enough apart to fit through the firewall. (See picture. The old heater core is on the right) I ended paying a radiator shop to move one of the tubes from the old heater core to the new one and now it fits.

My question is, I've seen comments on the web about installing something to reduce the coolant flow to the heater core to prevent the heater core from blowing out again. What would be the best way to do this? I would appreciate any input that you all might have.