You need the space, apparently not having it can corrupt the encrypted files and you definitely don't want that.
I'm installing to my HDD for now, once on there I can then move the GTA5 folder over to my SSD Steamapps folder, uninstall game in Steam (but first copying to another location just in case) and then reinstall game in Steam; Steam will find most/all the files in the SSD Steamapps folder and will be good to go as if it was installed on the SSD from the outset.
You definitely do need double the space on the drive to install an encrypted game to the same drive (if you can even chose where to install from encrypted - never had this problem until now)