| To stop the fuel pump assembly o-ring from rolling out of the groove, put plenty of Vaseline around the seal, groove and around the hole in the tank where the seal sits. Its always a little tricky, but this helps keep the o-ring in position. Don't worry about any excess getting into the fuel because the fuel will dissolve any excess Vaseline that gets into the tank.
The o-ring gets stretched when you install the assembly making it very difficult to reinstall. So it's best to buy a spare before you need to remove the assembly.
If circumstances require you to reuse it a second time (even if you break it), use a razor to diagonally spice the ends so as to take the slack out of it 'till the o-ring just wraps around in the groove - not too loose or too tight. After cleaning the ends (with say carb or brake cleaner), apply a little Superglue and then hold the O-ring ends together until the glue sets. Let dry and reassemble. | 
Shazaam!
Threaded Mode