E27 has a good point -

I have a few shell holders that have been modified, some with spacer shims that allow the rims to sit closer to the cuff, some with the faces thinned, some with both little modifications.

Careful with the thinned shell holders.
Make double damn sure you've used enough case lube at the bases so that you don't stick a case in the die and pull the cuff off the case holder.
It happens.
When you thin the face of your shell holder, you're also taking the case hardening off the part too.
They will break.

Cheers
Tinker