I Just did this
I'm actually waiting on my new tranny from Bruce to arrive (should be a couple more days), but I have new -8AN lines from the tranny to the coolerS. I run from the tranny to the cooler in the Radiator then to an external cooler then to a second external cooler back to the tranny. The -8 lines and fittings are DAMN expensive though. I plumbed new fuel lines and tranny lines, and spent over $1000 in lines and fittings. I think I used 1/4 to -8 AN fitting for the tranny and the same for the tranny cooler in the radiator. I'm going on memory here, so I can't guarantee they are 1/4 npt fittings on the cooler.
I HAD to do the tranny lines since I had one that I broke a while ago and used a rubber line in it's place. Well, it turns out that you're not "allowed" to have more than 1 foot of rubber hose on the car for fluids. So I did it so that wouldn't become an issue again.
Derrick