I'd order the 10....that way it will drop right in and when it reads "E" you will still have a bit of reserve in the tank.
Installation is not difficult, but this is a job that requires either hand or air tools only. One spark from an electric or cordless screwdriver and you can end up with a really bad day.
Pop the inspection plate...give the bolts a squirt of penetrating oil...let them sit a bit, then remove the bolts. Disconnect the two wires and the sender lifts right out.
I'd replace the gasket (and WEMA should include a new one)....drop the new sender in...tighten the screws....reconnect the wires....presto.
Two suggestions:
1.) Sender should be tight but not overtightened.
2.) Run the fuel level in the tank down first before you start this job. If the tank is full you will be dealing with sloshing fuel coming out the top of the tank as well as fumes while you are doing the job.