Thanks Danny. Back when I was working, I had a rudimentary understanding of all this stuff, because I used to hang tons (literally) of speakers above people's heads. Not so much now.
OTOH, there's this. When I started pondering, I got worried that, unlike most 3" lap belts, there wasn't a certification label on them anywhere. But they were in the installation instructions
As as for the suspension, it just as well. I realized it was a lot of work to reinvent the (rear) wheels. I'm going to go with SwayAway adjustable spring plates and a front sway bar. BTW, either you or Jimmy mentioned that a standard Beam sway bar won't work on a Spyder front end. Is that true?