Model and year would be helpful.
If it's N/A you could swap in the LSD from either a pre89 or 89+ Turbo model ad they're both taller. With the pre89LSD you stretch out the power band by about 10% and your speedo will need to be recalibrated for the difference as well. The speedometer drive gear from the R154 doesn't fit the W58.
If it's a turbo and it's a pre89 you could swap in the LSD from a 89+. The LSD from the pre89 is 3.91 and the LSD from the 89+ is 3.73. You just need to swap the speedo drive gear in this case to correct the speedometer.
As with any car making sure everything is running up to spec will prevent excessive fuel consumption. This includes: spark plugs, plug wires, distributor cap and rotor (N/A only), vacuum hoses, O2 sensor, temp sensors, and so on.
Before or as soon as you take delivery of the car check for any diagnostic error codes stored in the ECU. This will give you a heads up if anything's well out of the norm. Instructions and code descriptions can be found here:
http://www.cygnusx1.net/Supra/TechTi...ror_codes.aspx