We are in luck with the FUSO and Isuzu .
They use a particular type of seat belt ratchet tension system that can be used with spring suspension type seats. Fuso recently started to offer a BASIC suspension seat as standard equipment in some countries for the FG.
Interesting! Thanks for the info, EC. I was actually thinking about integrating a limiting strap element between the seat frame and the body mount, and then attach the lap belt to the seat points. I'm sure that would work fine, but it sounds like it's a non-issue from the feedback of more experienced folks. Thanks all. Of course, proof will be when they're installed, so I guess I'll see if it works for me. If they don't, several different ways I can approach it. I'll keep the build thread updated.
ETA: Well, just unboxed the first seat base. National supplies two pieces of webbing that attach between the stock belt attachment points and the new seat, which has its own belt attachment points. So they did exactly what I was thinking about doing....included in package. Yay, go me, woot. Less work for me.