A kind reader saw my seatbelt predicament and sent me a very helpful note and some photos, which he agreed to let me reproduce here for anyone else who's facing this issue. Thanks, Jim!
Hi Matt
I have recently installed the Hooker 5-point harness. Three comments with pictures may be helpful.
1. The attachment hole in the plates in Van's crotch strap kit needs to be placed lower to allow the steel shackle to sit correctly in the opening in the plates.
2. The attachment plates in Van's crotch strap kit are 1/8" apart. Although this is the correct spacing for the shackle, the Hooker strap is thicker than this and consists of three layers of webbing sewn together.
This pushes the front attachment plate further forward.
I had to make the two front screw holes in the seat pan oval to allow the screws to line up with the front attachment plate.
3. The length adjustment buckle is too wide to fit through the hole in the seat pan.
I had to widen this slightly.
Jim Talbot
Tauranga
New Zealand
This is pretty much exactly what I had resigned myself to doing, after I called Hooker and they had no help to offer me, so it's great to have photos to examine while I lay things out – this weekend when I have some time to work on it, that is.