5 point harness in a car w/o roll cage...How to do it?

For the guys that have a shoulder strap mounting points from the factory but the factory straps are either missing or wornout, I always wondered if you couldn't just use one of the racing shoulder harness straps mounted in the factory shoulder mount, at least it would be a three point instead of just a lap belt.
The lap belt could be adjusted farther to the inside so the shoulder beld would go farther across your body like the factory ones do.
Any thoughts on this?