pita has some good points.
I used to drive Extreme Rock Crawling 4X4's with a full in cab cage.
Now, I am not going to trust something mounted to the floor boards. If you don't have a cage, the mounting, IMHO, is not worth the money vs your safety. http://www.autopowerindustries.com/
These people make some real nice stuff. I ran thier 5 point system, with the in cab cage, PRP Suspension Seats and full cage in my Offroader.
In a roll, it was the best mod, besides the cage, I ever bought!!
Trust me, it's a major pain to get in and out of the car with the belts in a daily driver. I drove my truck while it was still Street Legal. Most people I have seen, including my self, used to just "flip" the shoulder straps over thier shoulders for comfort and ease of getting in and out of the car. Just defeats the whole purpose in my opinion.