MarkCAN
Last year I replaced entire exhaust system ( down pipes and crossover, resonator, muffler and all connecting pipes) 1970 MB 280 SL. Muffler was rusted out but resonator ( centre muffler) appeared to be in re-useable state, as it sounds like you just need it for out of car engine testing? Q? Need any other pipes?
See pic ( I hope).
Price: FREE ( fellow Canadian discount 🇨🇦) plus shipping from Invermere BC, Canada .