Want to flush old oil out of trans. and converter. Is it the upper or lower line on the transmission that flows fluid to the cooler?  

drop the pan... pull the plug if you have one...  some powerglides had them... drain the oil.. put in a new filter...   measure the oil that came out and put in 3 quarts ... start it.. check the level.. bring it up to full...    with the 4 or so quarts that you put in fresh.. that is almost half the fluid..      

cutting the lines like they do in quick service shops puts another leak source in...

you can run it with the line open... the transmissions really were not designed to flush them that way...