273 overheating after new timing chain set and waterpump

Something else I'm sure happened to me:

Oldmanmopar has claimed that over time, the tubes can crack loose from the fins and lose heat transfer. The rad can flow lots of water and not radiate very much heat!!

My original 273 rad would not cool a 318 or 360. Rad shop claimed it would "cool anything." It is the factory 2 core. I finally swapped in a later 73-74 2 core slant six rad I had and the temp instantly dropped a LONG ways. I'm convinced the old rad suffered this fate