spinner trimming technique

I'm finding new aluminum spinner installation a little more complicated than
originally thought G.

My new spinner shell extends rearward about 3/8" past the aft bulkhead edge
necessitating a spacer between the crank flange and aft bulkhead to clear
the cowl nose as well as then requiring longer prop bolts.

My question is: what fairly simple, non-complicated method or technique
could be used to trim off some of the excess?

As a side thought, does the spinner fabricator somehow ensure the backing
plate fits snugly and at 90 degrees to the center of the shell?

Thanks, Dick