Automags Online - Resources: The Level 10
Shimming refers to fine tuning the level 10 and ULTs with thin shims. AGD instructions will guide you through the steps when necessary.
Don't trim the trigger pin, don't adjust the trigger rod length unless its not at it's proper length, and don't put spacers in the rail.
There is probably about a 1% chance of the ULT reliably working in your classic valve. You'll get a lot better results from replacing your stock on/off with a RT on/off.