(pics to follow once I have a nice collection)
The error was too great to fix by trimming the wooden spacers, so I made a new pair. After my second trim+refit with them, the error is now under 1-in. Since the carriages (assemblies with the wheels) have 1/16-in of clearance to the rail-sides, my "wiggle room" out at end ends of the arms is pretty small, perhaps 1/4-in or so.
In order to be able to increase the width between the rails, I'll need to do a little trimming on the platform sides. Then I can fit up some more spacers, increasing the width until I get the arms fitting well on the studs. It's tedious and time-consuming work, but I only need to do it once, then can get on with making/welding up all the support pieces.