One torque spec I found for the nut on the transmission's oil cooler pipe was given in the 4L80E section of the 1998 GMT400 service manual. There, they say 20'#.
Click on this image to expand it for detail, look for "Transmission Oil Cooler Line to Radiator Nut"
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Here's another one, from the 700R4 section of the 1989 GMT400 manual, which says 21'# for the line to the transmission and 27'# for the line to the radiator.
I'm not sure I would be comfortable putting 27'# on that fitting. Using a backup wrench on that Dorman part, and applying 27'# to the nut on the transmission tube... well, I might try it. I would try 20'# first and if it didn't leak, then "done". If not, then maybe squeeze it tighter, but use 27'# as a maximum.
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