I had that happen with a Scott Drake trunk weatherstrip.  Sadly, the one company that used to make the best westherstrip now sells crap thst does not fit.

I removed the SD stuff and replaced it with cheaper variety and it fit fine.

It is truly sad.  That name used to mean quality.  Now its even contaminating the '79-'93 market with junk.  It seems Daniel Carpenter is turning out some really nice stuff by and large, and crosses in the classic market, but the offerings are few at this time.  Hopefully he expands and continues to offer the quality he's know for now. 

My deck lid has a dent above the liock cylinder thanks to that weatherstrip.

When I closed the trunk, it was so tight and raised 1/8” all around that the key could not turn the lock to open it. I had to apply extreme pressure while Sherry worked the lock in order to open it. That resulted in the dent.

Buyer beware!

I had had great luck using the SD windshield gaskets, but the one for 66 fastback rear window was oversized making it nearly impossible to get the stainless trim to clip in place without denting it.

I try to use Daniel Carpenter weather stripping - get it from Kentucky Mustang.  Never been disappointed.

I bought my 70 Mustang in 1978. Prior to that I had a 55 T-Bird. I those days parts were hard to find for it. It got in a accident and needed a new fender that was not to be found. That is one of the reasons I got rid of it. Back then I knew that Ford supported cars for about ten years so I bought all new weather stripping for the Mustang to keep till I needed them. Ford doesn't do that a much any more. I kept them in the house till about fourteen years ago when I replaced them. They were all in like new condition but they didn't fit like the originals. I had to file out the pre fabbed holes to elongate them so they would fit. The windows damaged the seals as originaly even after I went to the trouble to prevent that, I thought. They are just as supple as the originals.

Most of the NOS rubber products I have seen are hardened with age, when you remove them from the packaging the folds are pretty much permanent.
A new repop from O/E tooling is a better choice.

Where on the door do the seals not fit? On my 70 I had to file out the holes oblong to help the let the seals slide out toward the windows. My seals were made onto a metal band. Filling that a little is not noticeable. Just don't over do it. I also had to realign the windows. Doing that is a tedious job. They adjust in and out at the top and bottom, and move forward and toward the rear and also tilt fore and aft. On the my door the seals glued on so I put them where they needed to be to get the job done. I didn't need to reposition the doors but my doors have never been wrecked. I have had them off the car though.

Anybody have any experience with Steele Rubber Products? www.steelerubber.com