Here's the advice I was given by an old Cobra racer who still vintage races the originals. This has been discussed here before.
The proper place to attach the shoulder harnesses is the roll bar. There is a kit that bolts to the roll bar with eye bolts to attach the harnesses. Picture here:
http://www.cobracountry.com/cobra4sa...b-1332x578.jpg
There should not be a shoulder harness attached to the passenger side. First, the cross member is not strong enough once penetrated to support the G-forces associated with a crash. The tubing should be the diameter and strength of the driver's side roll bar. The cross member in the trunk is not. In practice it will provide SOME g-force absorption from a head front or rear collision but probably break before all is said and done.
The second and biggest thing about the passenger side shoulder harnesses is a rollver passenger safety issue. Should the car roll the shoulder harnesses hold the passenger upright, and thus, they become the passenger side roll bar. Without the roll bar the passenger will "fold" to the center of the vehicle. It's still gonna hurt like hell, and the chances of survival are low, but if you have shoulder harnesses the chance of survival is approximately zero.
My passenger side harnesses were attached to the seat bolts. They were on the seats when I got the car before I saw another car with them attached to support across the back and had the above conversation. This is equally bad and was done before I had the conversation above. The harnesses are still attached to the seat bolts but noone is allowed to use them (except for the 44oz diet coke I get when I go for a ride and hold them with the harness
) The next time I need to pull a seat they will be removed. I took my rear mount kit back.