I had the same issue when I replaced the standard silencers with the TOR ones.
It is the remains of the old gasket in there, welded tight for sure !
I stuffed a rag down the pipe first, leave an end out obviously then split the old gasket carefully with screwdriver and small hammer taps. The new gasket should fit easily to it's full extent into the link pipe so if it wont go fully in then there is still old stuff in there.
Every so often I vacced out the bits, finally pulling the rag out carefully..I did a brief start up to blow any small bits out, ten secs maybe.
It's HARD by the way so you will need to persevere