What is the best way to secure my sierra harness. Are cable ties ok or is it best to use p-clips.

I think the "recommended" way is P clips if you can find them big enough. I actually used cable ties threaded through cable tie bases which are riveted to the chassis. There are self-adhesive versions as well, but these are frowned on because they are not "permanent" enough.

Mine are like this (but not from here):

I nearly failed my SVA 'cos I used self-adhesive tie-wrap bases in some places. The tester and I reached a compromise...

David

You can use screwdown cable tie bases and cable ties, Quite neat and easy to alter should you want to add another loom to the run.
Steve.