The only real gain in using a 50 rather than a 20 would be in range and some payload increase but certainly you cannot increase the size of the freight itself(that is important too). On top of that a cargo door conversion for any of those planes rather than a 20 you will need to go through certification process(STC) and that is very costly and not enough investors or interested parties; so why go through the the trouble of all that if there is not even the business need of such fitted airplane?
Ultimately would you cut a hole in a fuselage of a beautiful 50 or 900 to load in some dirty freight???
