That cladding does look great. This kit-of-parts method of constructing these arched windows seems an efficient method: despite all those seams. A bit lego-like, but I like… 
Imaging the massive hauling & crane lifting work if they were done as ‘monolithic’ pieces - as done over at the other arched windowed building on William Street.
The windows casings at 520 5th may not be ’solid cast stone’ - but no one (but us) will know…. 
Here is a look at the SOLID cast stone done at 139 William street.
