Same problem I had. Took mine to get rodded out and was told the core was shot. They quoted me 800-900 dollars for a new core, (I think they're 4 row cores). That was about 5 years ago. Looked around and tried to find one but didn't have any luck. I ended up using a universal aluminum unit from Griffin Radiator. Had to fab a whole new cradle to mount it in the Mack. The only original pieces I used were the pressed sides. Took a good bit of work but seems to cool the 427 just fine. It will idle all day in the summer heat at 170-180. I'm sure there are places that could have repaired it but, I didn't want to fool with shipping it around the US.
I am on the hunt for the following parts for my great grandpas 1946 Mack EGX
Interior anything interior
I am told that I would have better luck being struck by lightning than finding parts for this truck. It has been a lot of work, but it is my grandma's dream to bring it back to life!