[Menno]

A quick sketch made in paint (sorry, will never use MS Paint again : p )

The angle iron on which the bearing-carriage runs can be bolted or welded to the support beam (or a piece of wood). I think this way it'll be more easy to make a rigid machine.

Most angle profiles are hot rolled, and need to be sanded for a smooth surface. I've sanded a little piece of angle iron and i must say the surface is quite smooth, however I don't know how straight the profile is after sanding it.