[Gav]

Baz
I have my trans cooler in that lower portion of the 'fish mouth' and have not had any temperature related problems even on track days.
Mine is an 'Earls' cooler which is loads thicker than a standard trans cooler.
The question is are you running a stall converter? If yes, the stall will increase the heat in the box due to slippage and a more cooling will be required.
If you are concerned you could fit a second cooler on the back of the radiator in series with the one in front.

There are a couple of pics of mine in my gallery.