Been there, done that....
Bought my CB350K4 with repaired mufflers, managed to buy a better-repaired set off ebay.
I did also manage to buy the muffler packing and indeed used it (goes sandwiched between the header and the muffler). Seems to be some asbestos-like material.
Finally bought the DSS replica mufflers. They look the part, but they were also too long for my bike. They wouldn't slip deep enough to align the mounting bracket with the footrest bolt.
I coudn't see any option but to cut the muffler and to extend the slot, both with the angle grinder. I knew it would leave the bare metal vulnerable to weather but I could use some zinc chromate on that. If I didn't extend the slot, the muffler would not clamp to the header irrespective of the clamp tightness.
That fixed the mounting stumbling block, but only in half. The asbestos packing would not fit between the replica muffler and the header. Not even by carefully cutting away half of the thickness.
I finally put a couple turns of adhesive aluminum foil, and once the mufflers were in place, wrapped the whole clamp area with more aluminum foil, and then clamped them together.
Come to think of it, I'm having issues with inconsistent idle and it could be because of a leaky muffler joint... but I would say the aluminum foil covered the area pretty tight.