if any and as many as possible that can take a look at your 1300 final drive mounted on you bike....... would you mind telling me the 3 letters that are stamped/raised on the inside of where the final drive and drive shaft bolts together with the 4 bolts? you would be looking at it through your rim from the disc brake side, you should be able to see these letters on flat inside of the FD housing, its start with the letter 'M'
being that the drive housings are interchangeable I would venture to say that the lettering is just a manufactures code so if there is a problem with housings they would be able to trace them.
If you went to the 1800 final at around 70 you would be turning 300 rpm faster than you are with the 1300 final, the 1800 final on the 1300 also makes it quicker off the line.
The guys with the 1800 want the 1300 final to lower their rpm at cruise by around 300 rpm.