Seatbelt anchor points

[TheArf]

Hi Guys just a quick question, is there any reason why when you fit the seatbelt anchor points they have to be in the vertical plain. I went to Loton Park hill climb at the weekend and I noticed that a lot of the competitors have them fitted horizontally, thus making the harness lie in the same orientation as the anchors and pulling in a straight line.
So my plan is to make stepped anchor points fitted through the actual cross member of the rollbar with a shoulder on them and fit them in from the rear of the car before welding and them in solid.