The 1999 and 2000 GMC sierra 2500 4wd with a 6.0 vortec do not have identical replacement oil cooler lines available from GM or aftermarket. The replacements offered required changing the radiator fan shroud, along with other changes.
Seeing that the changes required to replace the old lines would involve unnecessary and considerable expense, I decided to modify the oil lines supplied by the local NAPA store, that were supposed to fit my truck. They were several inches longer than needed, and would not fit for that reason.
To adapt the lines, I laid the new lines on top of the old lines on a bench and determined that the difference in length was 6 and 1/2 inches on one line, and the other was 7 and 1/2 inches. I used a tubing cutter and removed the excess line from each new line on the end toward the radiator, being careful to leave enough straight tubing at the cuts so that compression fittings could be used to join the sections. Also, I reamed out the tubing at the cuts so that there would be no restriction in oil flow. I made sure all aluminum particles were blown out of the tubing before installation. The tubing is 1/2 inch OD so standard compression fittings fit perfectly. I did not fullly tighten the compression fittings until the lines were properly positioned and installed in the radiator.
The only problem with the installation was that the new lines did not pass through the hose loom on the fan shroud, but they were close. This was a minor glitch that did not affect the installation.