Anything around the 160 mark will be good for your weight. When you said standing on the inserts, were you standing centred on the inner most set of inserts on either side? Or did the board only have 4 inserts under each foot?
Most/all boards in a 160 should have inserts that fit your stance width. I ride a 159 and a 160 and I'm only 5'7", I also like a relatively narrow stance and I have no trouble finding boards that fit.
Another option would be to buy a Burton snowboard with the ICS/EST channel binding mount system. You essentially have a near infinite adjustability with those, but again you should have no trouble riding at reference stance width on almost any 160 cm board.