I'll go along with Kadee shank springs and if you want to try one more thing before you do the full Kadee replacement, take your side cutters to the fixed side of the Atlas coupler, take a 1/16" or so off it and file it to a smooth edge again. Because the Kadee head is angled (non-prototypical ... but good!) it doesn't fit in the Atlas knuckle (the Weaver was better wasn't it), but taking that little bit off opens the knuckle up slightly.
Jason