Hi there, before I had raised the question, we had attempted to use the Add geometry attributes tool. However, when we changed the projection, it would not honor it. You can set the output value and it would still calculate the length in the original feature class projection. Based on this, we ended up creating a variable of a spatial reference (SR) in model builder, and selected the desired output coordinate system. In the "calculate length" (data management) tool, we added the following expression: !SHAPE!.projectAs("%SR%").length and that seem to do the job.
Thanks!