On GMC engines, from the port under the distributor plum to bottom center of filter assembly. From side of filter assembly, plumb to "T" junction block in port on lower left front of block next to timing cover. From other side of "T" junction block in port on lower left of timing cover plumb up to port in head on right side of thermostat housing. See photos below of 53 4 me's engine I took a couple of years ago for exactly this reason.

[img]http://www.oldgmctrucks.com/photos/stovebolt/gmc_oilfilterlines1.jpg[/img]
[img]http://www.oldgmctrucks.com/photos/stovebolt/gmc_oilfilterlines2.jpg[/img]

http://www.oldgmctrucks.com