P3690456 and 7 with the extended front hangers are the ones you want for a mid 60s b body application.

Unfortunately since time immemorial they've been marketed as "3400lb".
Please do yourself a favor and ignore that LOL.

The ones marketed as "3600lb" are really not for a B body at all.
They are for an E body, and the rear segment length is too short for B body applications.

SS springs seem to really be designed to work with specific body types. Any weight considerations are (strictly my humble opinion) red herrings applied by marketing folks from a very long time ago. When they applied the various weights, they took something that is actually very simple and made it unnecessarily complicated.

They're made in Mexico at the same spring plant that has made all the Dodge Ram leaf springs since like 1991 or whenever it was Chrysler pulled that business from their old spring plant in Canada.

Plenty of folks have had trouble with them, plenty of folks have had success...Ron's car is a great example of the latter....good luck.