The cooler unit is a loose fit inside the end cap of the radiator.
Transmission lines screw into this cooler and you can apply a little pipe sealer suitable for oil lines onto the threads.
When you tighten the trans lines it pulls the cooler tight against the inside of the radiator ends caps which are high density plastic and pretty strong.
When you tighten up it pulls the black rubber seals hard against the inside of the end cap to keep coolant jacket sealed.
The O ring is infact a flat rubber seal and you can remove and replace these easy, remove the inner plastic wheel arch for easy access.
Drop your coolant and remove trans lines, pick out old seals and replace.
Apply a little vaselene to keep them in place untill you have it all hooked up.
If it takes you more than half an hour, hang up your coveralls and sell your tool kit