That sucks. The only way for coolant to get in the trans was through the radiator so one of them did it and water in it will take it out. I'm afraid this is one of those times when a cheaper radiator ended up costing more.
You may want to bite the bullet and look into a Mishimoto or another quality all aluminum one.
My Mishimoto (2 core) radiator cost $380 and some change plus another $160 for a set of silicone hoses. Pricey, I know, but I don't want to have cooling problems leave me on the side of the road on a hot, humid Houston summer day. Hang in there, hope it all works out for ya. Keep us posted.
1-7/8" ARH LTs, 3" Magnaflow exh.,Spec twin disc clutch, 402 stroker , Fast 102 w/Nick Williams TB, 1000 hp shaft & axles, cam, fuel & suspension upgrades. 500 rwhp, 484 rwtq