last truck I changed auto trans fluid on was a 89 F250 with a C6.
I used a galvanized water tub as a catch basin. The nice part of the C6, was the pump pushed the fluid through the trans then the cooler, and back to the pan. Fill the pan, disconnect the return line from cooler to pan and start engine and start adding fluid, while helper slowly moves the gear selector through all the gears. Once the fluid coming out of the tube bright red, stop engine, connect the cooler line and top off fluid. As close as 100% change you could get.
Would need to research newer trans to see if it would would work without harm.
