Hmmm. 12 hours? I've been getting ready to do the exact same job on my 84 300SD, as the oil cooler lines are weeping a bit of oil. I also plan on replacing the oil filter housing gasket.
Did the threads on your oil cooler strip when you removed the lines? I hear that is a typical problem. I don't want to replace the oil cooler (they ain't cheap) and don't want to mess with brazing on new threaded fittings.
How was the access on the hex bolts that are used to mount the filter housing gasket. I have heard that it may be necessary to cut down a hex key to get it to fit in there. Did you encounter this?
1984 300SD 137k