WGR the plan from the source file to the target file. In the target file, make the new reference file invisible and create a new, visible layer. In that layer create a Layer Link (View menu) - choose the reference layer and select Project 2D Objects.
Set the scale of the layer w/ the Layer Link to what you want. Unlock the Layer link to move, rotate, etc., it.
Not using Viewports, are probably other ways to do this.