Been looking for a weatherstrip for my 78 B300 sliding side door for years. I remember seeing one a long time ago but it was $80 so I was holding out for something cheaper and never found it. I'd pay the $80 if I could find that again. DM me if you find anything!
Also why couldnt you get the universal to work? I've almost tried installing some myself but didn't want to be out $30 if it didn't work.