One of the things we look over pretty closely in an annual is the manner in which the seat is mounted in the car.
There are two paths one can go down. One requires a support for the seat back, and one does not. Seats with FIA certification do not require a support in back when they are mounted on the seat manufacturer's FIA approved mount. So if you show me a Sparco FIA seat mounted using a Sparco FIA seat mount, you don't need the brace going back to the supports in the main roll hoop. Otherwise, you need a solid support and it must be attached to the back of the seat (not just pressed up against the back of the seat).
The underlying issue behind the support rule is that some years ago (perhaps a decade) there were two fatalities in quick succession at a pair of non-SCCA events in the northeast (If memory serves, they were at Lime Rock and at Pocono.) In both cases, an unsupported seat was mounted to OE type toothed sliders. In both cases, the car backed hard into a solid structure (guardrail at Lime Rock, the wall at Pocono) and the teeth in the sliders sheered off. Both seats slid backwards and the drivers helmets got caught up in the roll cage and bracing.
As a result, the support rule was made universal and strengthened (previously we had not always required attachment of the seat to the support.) The FIA seat & mount exception appeared a few years later.
Some people play games with the support in order to retain front-to-back adjustability. I generally look these over pretty carefully; the adjustable toothed sliders are the reason for the support rule in the first place and a questionable adjustable support doesn't help matters.
The supports should carefully avoid the "spear pointed at the spine" design; we see these occasionally and usually end up spending some time explaining things very slowly and carefully to the drivers who think these are a good idea.
The support in my own IT car is perhaps over built, but I know it's not going anywhere. It consists of a 12" x 4" 12 gauge steel plate with two 1" steel boxed tubing sections welded perpendicular to the plate. These boxes go back and are U-bolted to the lateral bar in the cage that also provides the shoulder harness mount point.
For the bolts that attach the seat and the support, I recommend using a round headed allen type bolt on the driver's side of the seat, rather than something with square corners that will bother and possibly injure the driver.