1985's ran very well due to the trans, converter and rear axle ratio combination, try to keep it all original.

It wouldn't cost a lot to replace the filter and fluid including the fluid in the converter, plus add TransX to see if it helps. If it doesn't help flush the cooler and lines, replace the cooler hoses unless they're newer and pull the old trans to do a rebuild or get a good used unit from another '85.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer