Well, I'll just throw this out there. I know for a fact 68-69 cudas used a mixture of two slightly different clips when new. I'd have to assume it's the same on Darts. It may be possible to get by with a single size clip, I don't know.
The original screws used a dab of sealer on the threads. I found a couple virgin screws and clips under the carpet when I took my 69 apart. The original clips were kind of an olive drab color. It sure would not hurt to put a little RTV on the threads and maybe under the clips when installing them. Even when washing your car, the water will sit in the channel until it evaporates so it's good to weatherproof that area as best you can.
R/T Specialties used to be THE source for these clips and screws. They've probably sold thousands of kits by now. They give you the correct clips and screws in the right quantities but that's your call.
Jeff at jefframin.org had a diagram for the spacing on his site but it was for a cuda fastback. It would give you the general idea.
