Still not possible to convert HFS+ Journaled boot to APFS?
Awfully hard to test our software on a new file system if I can't convert the boot drive to the new file system


Since you have space to spare: convert an empty partition to APFS, then install 10.13 on the APFS partition. Or use SuperDuper or somesuch to copy your existing 10.13 installation to the new partition.