If you have the ability to do it, by all means do it. Much easier than putting it in & finding out it doeasnt work, or it does work for 5 miles before it eats itself & your converter also! Either way, your doing extra work.
If you cant rebuild it yourself, at least throw a new filter in it & inspect it. Might also want to replace the seals at each end, then you have the basics covered.
I wouldnt put a used mystery trans in a car without rebuilding it. They havent made TH350's for 20+ years, chances are its high mileage. Good luck!