Finally

John Morrow said
I had my old radiator checked out when this all started ............, he rodded it out, sealed off trans cooler said it was good to go ......... .....wrong.!

I seem to have had the same issue, sort of.

My factory 273 rad was checked when I had the 360 in the car, rad guy said "should work." NOT

Few weeks ago I "tossed" a 2 core 73-74 /6 rad into the car and it immediately cooled down.

Oldmanmopar posted sometime back something about he believes that cores can come cracked/ detached from the fins, reducing their heat transfer.

I believe that may be the case here. Both my rads are similar cores size, the original "supposed to be" clean. Yet, it won't cool a mild 318