So the difference I can see between 87 and 88 frames is on the use of lugs in the fork crown and the seatpost. This could be the reason for the different tube sizes.
About the rear brake cable, I moved it on left side of the steering (as the same as you) because on the right side (like shown in the catalogue) it was a bit constricted and was interfering with steering balance pushing the handlebar on the right side.
Tommy and Bowser could you post the under bb shell serial number of your Super Sierra? Mine is GZ 726164