Transmission cooler

More Problems….
Found some transmission fluid in the antifreeze after I drained my cooling system.
I assume this means that the cooler in the rad has failed.
I was thinking of just capping up the radiator and running an external cooler.
There are so many coolers out there I was not sure which one to use, what size, etc. (part numbers and/or links would be very helpful).

At this point I need to keep the cost down because the car has depleted my fun money. I am assuming that an external cooler is my cheapest route to get the car back on the road and have some fun before winter.

It is a 727 transmission on a mildly worked over 318.
Thanks in advance.