One of the members on here, Bob Carley, had Henry at Intermeccanica build a roadster with a Porsche 993 drive train in it. Bob also asked for a decent roll bar set up with seat belts similar to what you're looking for. Here is a video of that build and, while it's a bit long, you'll see how the car was built and how the roll cage is integrated into it. It's a really slick build.