If you're positive the rear seal was the correct replacement part, are you saying it's the real seal itself that's preventing the entire hub assembly from sliding all the way onto the spindle?

If that's the case, it could be that the surface area of the spindle that mates to the inside of the rear hub seal just needs to be smoothed over with some emery cloth. A little surface corrosion can sometimes prevent two such surfaces with such tight clearances from sliding together.