Yeah, Dave is right about metal lines being the best way to go, but if you insist on using rubber hose, buy hose designed for trans coolers, not fuel line. And to keep the hose from blowing off the existing metal line, use a flaring tool to put a little flare on the end, push the rubber hose a few inches over the line, and double clamp the hose.

I had to do this when I swapped the 5.0 Mustang engine into my Jeep truck, and it has been on there for 3 years.

If the part on the trans cooler is too short, buy a barbed hose fitting that screws into the cooler, and the barbs on the fitting will dig into the inside of the hose, preventing the problem.

Metal is the best, though.