Hi,
My 62 currently has fiberglass rocker panels on it. Great to avoid rust and about all that was available back in 1979 when I installed them. But there is no way to attach that retaining channel (too small for rivets, plus the heads would interfere). So I used automotive trim adhesive in an effort to keep the rubber seal in place. NOT a good solution! I found myself regluing it about once a year, or just living with the seal flopping around loose much of the time anyway.
Perhaps I could have glued the channel to the fiberglass, I don't know and never tried. The original channels were lost removing the original rusted rockers.
Anyway, I'm in mid-restoration and have steel rockers that will replace the fiberglass. So, I've also got the channel pieces to spot weld in place.
I'd definitely recommend using the channels, whether welded on or glued, as mentioned in an earlier response.