Has anyone found a way to change the 3/4 copper overflow drain pipe that goes into the galvanized pan (under a washer) to plastic flange assembly with rubber washers? The system that the builder used was to run a copper pipe up into the pan and then soldered it to the pan - but the pipe sits up over an inch above the base of the pan, severely restricting the amount of water that flow out if a problem arose. Thanks.

That 1" issue should not be a problem. The pipe will carry as much water as one flush with the floor of the pan, it just will always leav 1" behind. This only knick in when you have a leak so I would not be too concernec.