The 90 degree angle in the trim bends to accommodate the gasket.  At least mine did.  Probably worked better with OEM gasket, but I have never had one to compare to.  I went through four new sets of trim before I got one set installed with "not too many dents" in it.

I have used these folks rubber parts on a few applications, and found them to be right on:
http://www.steelerubber.com/rear-window-gasket-20-2848-81

Tim, no I have not.  Mine is not that far along, and it's a coupe.  I have used their stuff on my '83 F-150, and on customer cars with very good results.  On my own F-150, I had used window channel, and door seals from one of the big classic truck suppliers.  I don't remember their name, but I think it was initials, like L**.  Hint, hint.  Their stuff lasted about 2 years.  Changed out with the Steelerubber items, and 7 or 8 years later, still going strong.  As for customers, I have had no complaints, but don't really track the longevity.
As far as your issue, give them a call.  I have found them to be friendly and helpful.
Good Luck
 

Well, silly me.  I thought it might be useful information for the next 'Stanger that was gonna put the weatherstripping on the rear window.
Later.

Pablo wrote:

Well, silly me.  I thought it might be useful information for the next 'Stanger that was gonna put the weatherstripping on the rear window.
Later.

Pablo, A few weeks ago, I hosed down the 65 and had a dribble of water at the inside of the lower corner, leading me to believe I'm gonna hafta replace that 50 year old seal.  I was dreading the thought of hunting down seals.  You have offered an alternate to the stuff I know doesn't fit right, and I thank you.

A real plus is, that their website states that their stuff is "Made in USA".