I have a aftermarket cooler set up on my M-body.
I placed it in front of the radiator-same as you are contemplating. This really is the best/safest place for these.
Make sure the trans fluid outlet side goes into the aftermarket cooler first, then through the factory cooler in the bottom of the radiator and then back into the trans. This will insure the most cooling available.

Hope this helps,
CC.