The rear spar bulkhead also serves as an attach point for the seatbelt anchors. The anchors come pre-fabricated, but they need to be match-drilled to the spar carrythrough/bulkhead channel assembly. This gets done by laying out a mounting hole on one bracket, bolting it to the spar carrythrough, then bolting on the mating seat belt bracket clamped around a 3/16″ spacer. The entire assembly gets aligned at right angles to the carrythrough and clamped in place. The mating bracket then gets match-drilled to the bulkhead.

One picture is worth a thousand words…

Here are all the seatbelt anchors in place.