If the belts have new webbing, they are probably, as Michael said, as good as you're going to get. With non-retractable belts, the main thing is to adjust them so they fit you snugly. If you allow a lot of slack in the belts so you can move around easily, then in an accident your body can move forward quite a bit before the belts even begin to become effective, greatly increasing the chance and severity of injuries. Doing this restricts your movement in the car, but if you want the belts to work, you have to do it. Ask a race driver; they pull their belts up as tight as they can.
George Davis
'69 280 SL Euro manual