I just got through struggling with these drip rail moldings on my 72. My original drip rail moldings were removed and disposed of many years ago, but I wasn't too concerned because I purchased a set of new drip rail moldings from my GM dealer back then.
Well, 16 years later
, I get the car painted, installed a new vinyl top, without the padding the inexpensive tops have and try to install my new GM drip rail molding. No way would it go on without totally destroying them!
I reseached the topic here on Team Chevelle and see there may be a difference in the moldings between hardtops and vinyl top cars. So, I buy a used set that was supposedly from a vinyl top car, but they wouldn't go on either, so again I buy another set of drip rail moldings from another guy that just parted a vinyl top car.
When I get them I compared the molding to the other sets I had and it was quite obvious that the ones I just got from the vinyl top car were going to work. The channel that fits over the drip rail that the vinyl top covers were MUCH bigger, not just bent out, but much more curved to fit over the covered drip rails. I polished them up and they snapped in place, piece of cake.
Ther moral of the story is to use a set of drip rail moldings that originally came from a vinyl car, you'll be happy you did!