the stock trans cooler on mine is a stacked plate design like the b&m cooler
its very little restriction so id put the b&m one as the inlet to the hottest trans fluid and the factory one inline after the new one....
BUT it really depends on how hot you are getting the fluid
you need to get the fluid hot enough to evaporate all the condensation (from heating and cooling) and keep trans fluid temp below 210
if you notice temps above 210 then you need an additional cooler

my 67 mustang frequently has trans temps over 240 (3200 stall converter and i footbrake it a lot) and as long as you can cool it off in a reasonable amount of time, you are fine. my oil analasys of the fluid shows that it has not been effected very much with the extreme use. granted, this is in a strip/street car that may only see 3000 miles a year, not a truck that gets 12-15k a year, slightly different application