Does rebuilding a clustered index offline require extra space for each non-clustered index?

I have an uncompressed clustered primary key. It consumes 63.6 GB in the main part of the clustered index with 17.9 GB LOB. The table's only non-clustered index is 57.3 GB also with 17.9 GB LOB. I wish to rebuild the clustered index offline without the database growing. Running sp_spaceused with no arguments reports 66 GB in its "unallocated space" column. Is what I wish to do possible? Or will rebuilding the compressed index force a rebuild of the non-clustered index, thus taking at least 100 GB? Despite my best effort, I have found an answer in neither the documentation or Stack Overflow. My experiments with a small database that I had handy suggest that rebuilding the clustered index does not force a rebuild of the non-clustered indexes.