evidence: I suspect you will be on shaky ground legally speaking whichever type you go for as the law doesn't allow seltbelts to be retro-fitted to cars unless they were designed to take them (as in some 1960s cars where they were optional). You may of course take the view that your safety is more important than the letter of the law, in which case I would favour a 3-point belt over a lap belt due to the proximity of the windscreen!