As I understand an "H" style harness, they use a piece of webbing stiched between the shoulder harness straps to keep them together. The issue I have found with that arrangement is the H-strap contacts the back of the seat as the straps are tightened up, but the strap from the back of the seat to the harness mounting point is still loose. The problem is, you are not being held back firmly by the structure of the roll cage, rather, you are only held in by the strength of the seat back. I used an H-harness in autocross, figuring it would help keep the straps on my shoulders. When I change to road racing, I used the individual shoulder strap arrangement with a Bulter seat. NEVER did I have a strap come off my shoulder, even after testing it into a cement wall, twice. I now use a seat with holes for the harness, which makes it unnecessary to even worry about the straps coming off my shoulders. If you want to use a bolt-in style, have a plate welded to the roll cage at the appropriate point, and drill/bolt through it.