Seat and Seat belt questions

[Fluid Power]

Trying to get the car buttoned up and have started thinking about seats and seat belts. I bought some Corbeau's with a harness because I liked the looks. I didn't put any thought into how the harness mounted until I opened the instructions. I have a 69 camaro convertible, and was not going to run a cage or roll bar. Mounting the harness to the floor of the car goes against the mounting instructions for safety reasons. They would like to see them mounted perpendicular to the back seat (ie, to a roll bar cross bar)

Any body have any advice or suggestions? I really do not want to change the seats or put a bar in since the kiddies will be riding in the back. If I have to dump the seats for a better restraint system I will, but I am open to suggestions.

Darren