I just looked up Transmission Cooler Hose last night at RockAuto.com. They sell Gates Transmission Cooler Hose that is rated for 400PSI.
IIRC, most fuel injection hose is rated for 60-85PSI.
I don't think the line pressure gets that high in the cooler lines, BUT, pressures inside the trans can be in thousands of PSI. So, I think its a matter of pressure surges in the system can make their way through the systems sometimes and thus to the lines, so you have to have a pretty high pressure hose to ensure they won't burst over years of use. I could be wrong on that.
And its NOT only pressure rating, gasoline and oil hose can have different chemical make-up and using the wrong fluid in the hose than intended by its design, can result in weakening the hose, and then you'll get leaks or have it burst.
Fuel Injection "Clamps" have always impressed me as being a good design that would clamp the hose very tightely. Maybe NAPA has something even better. As well, you have to have some sort of crimp at the end of the line, if you're cutting metal lines, you can't leave it as a smooth line on the end. Something has to be there to prevent the hose from being forced off the end of the line. You could get a brake double flare tool, and stop halfway through the process, that would leave a nice buldge on the end of the line to prevent the hose from being forced off. Another, the ring for union fittings, either braze/solder it on the end, or just attache a union fitting, that will crimp that ring on the end of the line, then cut off the fitting.
You already have a filter in the trans, why do you need a 2nd or 3rd in the cooler lines? It might reduce flow through the cooler and heat up the trans, as well, the trans can be very sensative to line pressures, and a pressure change from a cooler in the line could effect the trans. Yea, unlikely, probably wouldn't hurt anything, but IMO, you don't need the filter, so why risk it causing any negative effect, even if its unlikely.
On the 5WA580/NAG1 trans, the output line is on the Driver's side, the return line is on the Passenger Side. This is the V6 Trans. I don't know on the V8 Trans.
"Typically" the higher hose from cooler is the return line, but NOT always, and I could be remembering it backwards, so you need to confirm it.