Regarding the du and df:
I have seen an old Unix man page that tells to look up the block size in the /usr/include/**.h files.
Not very user-friendly. In SysV it has always been 512 bytes. I think they wanted to have a small buffer in the kernel driver (and physical sector size was much below 512 bytes at that time).
du -k and df -k seem to be widespread in Unix and are allowed(ignored) in Linux.
But in Solaris, df -k not only changes the values to kilobyte, it also changes the (really bad) SysV-style output to the BSD-style (that BSD Unix and Linux always use).
Being Posix-compliant, e.g. by using my template, df -kP might be quite portable (in Linux even avoids a brain-damaged "split long lines in two even if destination is not a terminal").