good luck!

the 2.47 diff will make a difference in economy, you might want to get a Torque converter from an 85, it has a lower stall speed, for better acceleration with the tall diff...

the 85SD can get 30mpg with average driving, with the 2.88 diff. (I did it regularly in my SD) if you feel an IC is worth it, let us know the results, but I recommend getting the car solid before adding the IC, so you know what made the improvements.
get the diff, in and record the results of that, log miles, mpg, and times to 60/30/45, etc... then add the IC, and record the changes.
OR ad the IC first, and record the changes from stock, then make further changes.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!