Doing a final install of the passenger seat was a chore - in drilling the holes in the frame to mount the seats I had erroneously positioned them a little too far forward. As such, the bolts were up close to the vertical wall of the 2x4 frame tubing. The bolts went down through the floor, but putting/tightening a nut from the bottom required some custom "tools". I had to grind a deep-wall socket flat on one side and work it around. It took a while, but did get done. This shows the finished passenger seat with the 5-point harness completed.

Another word to the wise: the base of the frame is constructed of 2x4 rectangular tubing - including under the base of the seats. The seat studs go down into the top of the narrow side of one of these tubes. Make sure your holes are centered in this tubing (front to back), else you'll be grinding some custom tools as well.