Well, let me give you some more bad news ...
The PN you have is referred to as a STANDARD PN, i.e. it is a hardware number and can apply to many model lines. Your CHASSIS CATALOG gave you 358397-S while 54vickey came up with 358397-S8.
The trouble with these is that the parts catalog can give a PN, but when you went to the PRICE CATALOG (or O.S.I. catalog), it may have had a different PN SUFFIX as you have come across here. Add to the fact it was for the 54 model year, it is not likely you will come across it either NOS or re-pro.
There is another CATALOG, named the HARDWARE AND UTILITY CATALOG (of the correct time period) that will define and give a drawing and sizing of the fastener. By using this as a reference, it should be possible to find a universal clip (preferably nylon).
In your BODY CATALOG, does it give info as to whether the retainers were included with the replacement molding or had to be sourced with a separate CLIP PACKAGE? Some things were done differently in that time period.
I have a 1973 STANDARD PARTS CATALOG here somewhere ...

. Let me see if I can find it before you buy anything else.