A Google search located retrobeltusa.com and they list specific belts for your car, and all other '65 Chryslers too.
888-325-1555.
Seems you choose the length and whether retractable or not.
You'll notice the eBay ones say (fits 1965 Chrysler 300) so are generic ones which likely "fit" any car.
The Chrysler 300 club international chrysler300club.com may be able to help.
Hopefully some other forum member with a '65 can answer your query.