PNP LTSP is not working anymore on Ubuntu. I know because I used it prior to 14.04. However, at 16.04, it stopped working. Or, at least, not in terms of any of the Ubuntu instructions. Additionally, even if it was working still, PNP LTSP is best suited to a 32 bit host image because the client image is a direct copy of the host image. Given that LTSP clients may often be physically 32 bit, it always pays to have the client image as 32 bit. That way it will run on both 32 bit and 64 bit client machines.
But, Ubuntu is soon to drop 32 bit support from what I understand. Thus, PNP LTSP, even assuming I could get it to work on Ubuntu 16.04, would be no good since it would only give 64 bit client images. The only type of LTSP that would get round this is the traditional LTSP setup, where the client image can be either 64 bit or 32 bit. But, that version of LTSP has not worked properly on Ubuntu since 12.04 except where it has been built into the installation, as with Edubuntu. However, Edubuntu has just died at 14.04.