You're completely right Bob. Although I didn't mentioned it, I assume too that the seatbelt is complete (including the chest belt).
Anyhow, the waist belt will be the "anchor point" for the whole body, unless Neil has a fancy pre-tensioner installed (the regular ones, although they do a good job in holding you to the seat as close as possible, perhaps some more clearance to the driving wheel would be welcomed
On a side note, your observation on the physics of collitions is, again, right Bob. I just wanted to add something to your comment... you do want that the thing you hit has the characteristics to:
a) hold you to it (not only keep you bouncing, like you correctly mention with the seatbelt tightening)
b) be soft enough (or absorbing enough, for that matter) to not hurt you when that thing, along with you, come to a stop... because at some point that thing WILL stop (and most likely, it will be harshly)
Keep the great posts! And good luck with your very interesting project Neil!