I wouldn't attach the shoulder portion to the floor, because in a crash it could cause spinal compression = not good. They should come up over the shoulder and relatively straight back to an attachment point.
I've heard this even though I've seen MANY harnesses attached to the floor.
Here's a new question. Is there a roll cage that leaves the back seats intact and easy to sit in? As I do carry more than one passenger at a time on occasion.
