I had one of those! Same year!
People who have bought the rear mount STS turbo systems for Vettes and other V8 musclecars are experiencing mpg increases after the install and remapping of the efi system, so doing the Cressida should not be that much of a stretch. You did not say if you had an auto or a 5 speed in the car, but either way I believe 30 mpg is attainable.
I have been out of the import scene for a minute, so locally I don't know who is a reputable tuner with a proper chassis dyno. I do know this though; get yourself a J&S Safeguard knock controller and optimize the ignition timing. This thing will allow you to run a more aggressive timing curve for best power and mpg without rattling the old girl; you know those old 5M and 7M engines love to push the head gasket. Add water/meth injection as another level of safety.
I recently saw a Cressida done old school style: lowered, either Enkei or BBS rims, JDM location side view mirrors. Beautiful machine.