As I remember (been a couple years) on my '91 the big nut screws directly onto a protuding nipple of the trans coolant tank (not directly to the plastic radiator) with a rubber washer in between to seal the radiator. So..... you should be able to remove the small hose nut, holding a backup on the big nut, then remove the big nut from the trans tank nipple without putting undue stress on the radiator plastic. Replace or seal the gasket and gently tighten it all back up.
But remember, as you loosen the big nut, you are applying force on the copper trans cooling tank inside the radiator, and you don't want to fracture it; that would mix water and ATF, not a good thing. So, if it doesn't unscrew relatively easy, don't get too forceful.

Good luck,

DG