I just run everything cooler than stock, with a computer car I use a 180F thermostat and reprogram the fans to be all in by 190F. With the radiator running cooler, so does the transmission.
If your '91 is like my '88 you might be happy with the cooler I used. It might be bigger than what you want but installation of any cooler would be similar to what I did. Transmission Cooler Installation Instructions