It’s your current muffler that is be the problem, maybe it’s for a later SC, maybe oversized, race design…I don’t know, even still it shouldn’t be so crooked, maybe the muffler flange can be cut and fabricated to reduce its length, that’s the fast way to go.
I take it this muffler also fit crooked on your original bumper.
But again as stated by @RarlyL8 the F-bumper on a G-body have issues.
So going the route of complete exhaust change over may not solve a “too close” fitment issue to avoid heat damage to the fiberglass. Btw, you have the thin header flanges so any replacements will have to be such.
I’ll post this Old Pelican ForSale thread on what your system looks like laid out.
Notice the “straight pipe” in place of the pre-muffler (or catalytic converter if yours is U.S. California equipped) and the diameter of the oem stock muffler. Your current muffler seems larger. The straight pipe may not be TUV acceptable, so check on that if you were wanting to replace.
FS: 911 SC Exhaust Heat Exchangers and Muffler
I don’t think there’s anything to adjust other than that aftermarket muffler flange tail section if it was altered before. It’s a flange-to-flange set up, not wiggle room.
You may go out and seek a stock SC muffler - maybe some shop has one laying around that will allow you to try fitting it and then you can check your clearances with the new bumper on before going through on any replacing, cutting, welding, spending time and money experimenting.