I've started to converting my rolling stock to Kadee.
A few modifications to others here. So far I haven't used to many shims. I've converted AtlasO and Weaver cars to date. Only a Weaver Caboose needed shims. Instead of using 2 X 1/2 wood screws I've been using 2 X 3/8 where no shims have been required and with Philips heads not slotted purchased from Micro Fasteners. The 3/8 length seems to work best for the gondolas and flat cars where I don't want a screw to pop up through the floor.
The issue I'm having now is with my new MTH rolling stock with the new couplers MTH is using. There so much to remove to get clearance for the Kadee box it doesn't seem worth it. May have to purchase trucks with out all the parts.