I run a small trans. cooler (5 1/2 x 12 )on a warm 340 w/3200 stahl Dynamic converter.
I've been told the trans. should operate at 170 - 190 degrees.
When I shut it down I usually feel my lines....they are never too hot to touch and I usually get on it pretty good when I drive it.
If you have a gauge on your Trans.what temp. does it run?.....maybe I should plumb my cooler in series with the little pipe in the rad to add some heat,even though I only drive it in the warm months.......tx...al

My suggestion would be to put a gauge in there & see where YOUR temp is at for a baseline then decide if it's needs changing. I've read the posts about using the bottom stock cooler in series to warm the fluid but after reading about some of them developing leaks & contaminating the trans w antifreeze I've always preferred to bypass it & use only a large aftermarket flat cooler that comes w the (4) black plastic rods & no problems to date. that & I would not jump on the eng until it (& the trans) are warmed up.