My obs ext cab lb are 78" iirc and they end right at the back of the door. I have never had a single problem with them binding there. Stopped all wheel hop and didn't affect the ride much but I have a lot of leaf springs compared to others.
length depends on the axle mount location not the chassis length. Lower trucks will have longer bars than taller trucks. Get the bar parallel to the d shaft and you're good to go. That said, mine are not perfectly parallel and work as they are supposed to.