the Valhalla 360 doesn't work and isn't made to work with shoulder/seat belts. having used both types of seats in a race car and used the Valhalla device in karting, the Valhalla is the only way to go if you are using a typical kart seat. if you do use a oval stock car type seat with the head bolster, then you should use the HANS or once of the restraint systems.
none of the head restraint systems are designed to prevent or lessen side impacts. that's the job of the head bolster (and they do make one for a typical kart seat that should be used in any champ type kart).
have to agree with outrider in one respect....the HANS, the head, shoulder and leg bolsters as well as the rib protectors are all part of the seating system.....but in the case of kart racing, unless your racing a mini sprint or something similar that uses a stock car style seat...then get a halo from LaJoy, mount it to the kart seat and get a good restraint system....if you don't use this, then I wouldn't be on the track without wearing my Valhalla 360 device....