Coolers are in parallel now and the operating temps are staying down in the 180's in traffic in hot weather and lower yet on the higway at speed .
Cooler outside air temps & the trans temp hovers between 165 & 180.
I could install an on/off switch to the relay control power to both fans if in winter driving (mild here) the trans needs more heat.
If you have trouble getting the fittings through McMaster Carr Supply
Here are the two companies that make/sell the stanless steel hose barb wye fittings.
COLE - PARMER
ELDON - JAMES
The SS wyes are not available in an increaser/reducer pattern where the two branches combine to a larger single diameter.
That would be good for running increased line sizes in & out of the trans up to the split for the coolers.
A straight increaser/reducer fitting would have to be added in the run but the 3/8"x1/2" reducer is not available in stainless either (every other size is though LOL).
Edelman may make it in brass. Either that or some type of compression x barb or barb x thread adapters may be available to get to the increase.
Do not use plastic fittings . The heat & oil will kill them.
Remember that the branches from the wyes to the coolers have to be equal in length for obtaining balanced flow ( as close as you can get to it) through the coolers.
Bends in the hoses must have a radius not less than 3".
Anything less than that will cause flow restrictions ,possibility of kinks and place stress on the cooler inlet & outlet nipples.
Feed the coolers in upflow configuration : Hot- in at the bottom /Cool- out at the top.
This will help to prevent any gases/air from getting trapped in the coolers.
Up & out back to the trans (HVAC VAV re-heating coils are plumbed the same way).
In the pics you can see where I chose to eliminate the pass through hole on the core support center brace. It was ok for the series hookup but was causing too tight of a bend in the supply/inlet hose branch to #2 cooler.
I put some edge protector on the brace to protect the hose.
1/2" foam pipe insulation works well on the hose lines to protect them where they pass through tight areas with sharp edges or where there is the possibility of a hose chafing/abrading. Just cover the hose in those areas where it needs to be protected. You want as much surface area of the hose exposed to free air as possible so it too can radiate off heat.
This was the way to go for my setup.