The driving gear is a loose fit on the splines of the driveshaft but it is retained in place after installation of the bearing and the thrust washer utilizing a c-ring clip.
The driven gear is housed in a cap that is numbered with the tooth count of the gear. There are at least two different ratios of driven gears for different rear axle ratios. When the tooth count changes, the diameter of the gear also changes so the cap has to be able to set the gear in the proper location on the drive gear for proper mesh. Adding gaskets will reduce the mesh so I would only add a gasket if the mesh was too tight. Best rule of thumb is to make sure the driven gear tooth count matches the number on the cap. It's OK to have a little bit of backlash on the gears but you don't want too much. Just be careful when adding any more gaskets.
If the gear cap is not marked with a part number or a tooth count number, then a person needs to make sure the backlash is close. If it's too far off, a person should get a gear cap to match the gear being used. Most caps I've seen are marked. Ford used this set up clear up to 1948 so there is a lot of stuff out there in the way of parts.
