The seat belts and seat work together in an impact, if the seat is not securely fastened to the car the harnesses will not work as seat movement will allow slack, the belts hold you into the seat so that the seat provides the opposing forcde (Newton's law and all that)you risk broken collar bones at the very least.
Ari Vartinenen's horrendous injuries in the Argentinian Rally were mainly due to the seat collapsing and breaking so the seat belts effectively went slack.
You will also need to put in big spreader plates for the seat belts to attach to unless you use the original mountings.
As essgee said it's your ar*se
Hitting things with hammers until they work.