Please Help...67 Barracuda Rear Glass Problem

I have a very bad problem...im restoring a 1967 plymouth barracuda coupe and i was informed that nobody sells the rear glass rubber! i was informed that the 68-69 would work so thats what i got but i can not see how it can work! itsvery tight and slugish and the worst thing is i cant even get it in! so PLEASE...if anyone can help me with some info of what i need to do ...by all means please do so!!! Thanks guys