At no time should the belt adjusting hardware contact the metal seat. It must be on either inside, or preferably outside, the seat. Such contact can cause improper adjustments as well as the infamous "dumping" where a belt is stacked against one side of the hardware. Dumping could cause shearing against a hardware edge.
The shoulder belts must not depend on the seat hole for their vertical location. There should be a seat mount bar or chassis bar behind the seat that locates the height of the belts coming off the driver's shoulders. The bar should allow belt angles of about 10 degrees down behind the seat.
Most of us are familiar with the five-point harness; it has a single, front anti-submarine strap attaching below the seat. This isn't bad. I've used one for years, and my voice is still in the same range it has been in since I survived puberty. The purpose of the strap is not to keep you from sliding forward, but to keep the lap belt from riding up and then letting you slide forward.
The six-point harness has a double strap, if this makes you feel better. The best part--it's easier to mount than a five-point setup. The double straps attach to the lap belt mounts. Then they wrap under the seat and go up through the front hole in the seat.
Conclusions
Take time to properly position a seat; use a mounting that attaches in three places to the rollcage; pick a seat that fits properly; use the correct brackets; and finally, use care in selecting belt mounting points.
 The shoulder mounts are shown...  The shoulder mounts are shown in two ways. The best way, ifthere is room, is the double shear tab on the right. There mustbe room for the hardware without it touching the seat. In caseswhere space is not available, weld a single tab straight down off thecage crossbar. This location makes the belt pull in line with the tab.Never weld on a tab pointing straight back in this location. |
 This is a close-up of the...  This is a close-up of the six-point anti-submarine belt attachment. Itbolts to the outside of the lap belt mount on each side. |
 A bottom view of the seat...  A bottom view of the seat shows the two anti-submarine straps routedunder the seat and attached on each side at the lap beltlocation. The tab in the center is for attaching a five-point harness.This tab should point down or back, not forward or up. |