Leaf spring difference.

Pretty much. You have to have a '65 Parts Book. Then look up the Leaf Pack for each. Then if the p/n are different, look at the main leaf part number and see if they are the same or different. From that the differences, if any, can usually be figured out. The shop manual sometimes will throw in some clues as well.
You can also see what Stengel Bros, with one of the most complete aftermarket listings, shows on their website.
But even they do not have a complete factory reference on-line. And most of the others show the typical "one size will substitute for most" listings.

Chrysler used different thickness leaves, free arch, and number of leaves or half leaves to get the springs to do all the things they needed to do. I have no idea what you mean by stout. If its going to be used for drag racing with slicks, then a high ratio spring with a thicker main leaf is what will survive best. Extra clamps on the front can also help.