1: make backups of everything.
2: uninstall programs/apps on the prebuilt system until the total space taken on the hdd is under 300gb.
3: clone the 1.5tb to the 320gb
4: install the 320gb in place of the 1.5tb and boot up to make sure everything works. (If it doesn't then you'll be really happy with step1.)
5: use the ssd and 1.5tb in your new system.