Welcome to the forum.
The Stevens 320 is a Chinese knock-off of the Winchester 1300. There are
short buttstocks available for the 1300--whether or not they will fit the 320
I do not know.
If you want to retain the pistol grip your best bet might be to shorten the existing
stock. It's synthetic, most likely hollow, and probably a pain in the butt to shorten.
Not a DIY project for a total newb, but if you have some experience it's doable.
ETA: Just looked at the Brownells catalog. They sell a couple of adjustable
pistol grip buttstocks that fit the 1300. Cheapest is 019-000-013
Looks like they are made by this outfit http://www.atigunstocks.com/mossberg-tactical-stocks/241-six-position-shotgun-pistol-grip-stock-standard-forend.html