Using the cross bar between the strut towers can present an issue with the belt angle if you are using a Hans device - if too low it apparently puts alot of downward strain through your shoulders. Also, the longer the rear length of belt the more stretch you will get.
Is the car for competition? Have you consulted the CAMS and FIA info?
When designing my car I could not really find any facts as to whether rear eyelets were any better or worse than wrapping teh belt around the rear bar. I went with wrap around.