TT,
That collar that is welded to the headpipes is meant to lock in w/ the O.E. muffler clamps and as you accelerate,vibrate,etc. the O.E. clamp has a kind of hook shape to the end of it which catches the front edge of that collar and hangs on to the muffler/pipe assembly, so that even if you were to loose clamping down on a good tight exhaust packing(I've been looking on the websites/fiche,my parts,other members of forums & their measurements,etc. ...the search continues)as the O.E., your muffler would stay held on w/ that hook/clamp set up. How do the clamps look that you purchased ? are they similar to O.E. & do they have that hook/holder section that I mentioned ? I you need to, you can find many of those O.E. CB350K2,3,4,5 exhaust muffler clamps in a salvage yard off of a completely rusted out muffler pair...those clamps last forever even if the mufflers are disintegrated from rust;then you have a job removing the thread sealing collar off of the clamp bolt as I described in an earlier post.
If you can,post a close up pic of the muffler clamp that you purchased,also measure the front to back width of your clamp & then I can measure the O.E. ones I have to see if they are wide enough to allow you to move the muffler back to where it will fit w/ your rear hanger brackets.I can also recommend turning your rear hanger brackets over(remove them from the 2 muffler mount bolts & flip them over),I've done this before & it made up the difference in the correct fore/aft distance that was required.
TT, I have been on a roll w/ my research on those muffler packings & hope I can give you the good news tomorrow; make sure you have new head to pipe (round ring) gaskets also.
Bill