I have a '61Impala convertible and am also parting out a '63 Impala 2dr hardtop. The sections under the vent windows and the doors would be the same for '63 hardtops and converts. When I measure the rear quarter beltline moldings, my '61 convert moldings are shorter than the 2dr hardtop. On the convert, the back end of the quarter beltline molding tucks under the front edge of the pinch weld snap strips for the boot. I imagine you can trim the 2dr hardtop quarter beltline strips to fit the convert. Hope this helps.
All depends what he means by "Belt Line". I really hate that term. The Rear Quarter "Body Side Moldings", what I took that he was referring to, are one.
The other (at the top of the Doors and bottom of the Glass and continuing on to the Rear Quarter Glass) are called "Reveal Moldings". This is what GM calls them and what I have learned to call them through the years.
Hopefully he will jump in and clarify which ones he was referring to.
I agree that the description is confusing. When you google the term, the reveal molding (or "arm burner" molding, as it is sometimes called) comes up first. It would be good to know which molding he is asking about.