Exhaust seal/and MSD

Hello have a question to you again with my a8 3,7l aew motor bj97 the kat incl. Renewing the tube (evil put on) have used the parts from an accident cheaply got I have to know now what seals are needed for the cat on the driver's side who can help me.Then I have another question to the msd have one of the 4.2 l tube diameter60 mm my at the 3.7 is it 55mm I can use it with turned adapter pieces or can happen there because of congestion pressure or so, the previous owner has done whole work and simply the msd that. and himself tubes boge horny sound but unfortunately elegeal. hope for your help thank you