muffler suspension

i use to be happy with my muffler on my '84 308 but more than once now i have been adjusting (read lowering) the catalyst in order to raise the tail pipes so they will not touch the metallic housing at the back. now my catalyst is tipping as low as it can and i am now considering shortening the suspension of the muffler itself. there is an hexagonal nuts ( 17 mm) at the top end of these 2 coil springs supporting the attachments to each side of the muffler and i wonder if this would be a possible adjustment to compress these springs so as to raise the muffler itself instead of playing with all these donuts.
anyone has been there/done this before ? thanks for any answer.