They screw into the valve. You'll need to know what length you need first. Therfore, unless you can see the valve without taking the underbelly down, that should be your first step. You may be able to determine using a tape measure if you know where the valve is.
BTW.. if you know where the valve is and can't see it, you can cut a 3-sided access door in the underbelly and then tape it back up. Maybe you can see that by taking a few screws out and pulling down the underbelly to peer in there with a flashlight.
Pulling the whole underbelly down is a real PITA. Not so much getting it down but putting it back up into place.