Pretty sure your wrong about that. Same frame, same tank, same fender. They should all interchange.
The difference might be the dash or panel on the tank. I know that ont he older models that there is a difference in the length of the dash. The electraglides have a shorter dash too them which means the nose of the seat is a little longer. So if you put a street glide seat on an electraglide then you have a big gap between the front of the seat and the dash panel on the tank. If you take and electraglide seat and put it on a Streetglide then the nose of the seat would extend up and over the dash panel.