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.

Have a complete 48 EE I picked up for parts.  Has many good parts.  I am taking a stainless strip off the hood, the windshield crank handle, one fender liner and maybe a vacuum line bracket.  Has a reman engine and trans.  t's the green one in the photo.