I'm putting sm exhaust on a 1999 454mag mpi. I'm using risers that have been extended so the water mixes roughly 12" further back. Will I need to get it remapped, up the fuel pressure, or possibly be ok? I know with full tubular headers a remap is a must, just not sure on SM.

Ran them on my 502 Mag MPI with no adjustments or problems. I did a loaded run, shut it right off and plug read just to be safe. You should do the same. Good luck and enjoy!