If it were me, I would contact someone scrapping out M939 series trucks as mine has a magnetic drain plug from the factory. To get your old one loose, use a hammer and chisel on the edge to get a bite and go the correct direction. Righty tighty, lefty loosey has been a good standby for most fasteners until you get to the driver's side wheel nuts. Make sure to have at least 2 empty 5 gallon buckets ready to catch the oil. Once you get the plug loose enough to leak pretty good, place the bucket where it will catch the majority and walk away for about 30 minutes unless you've warmed the engine up first. When it comes time to remove the oil filter, be ready to divert the oil from the top of the axle somehow or clean the driver's side of the front axle afterwards. If you still have a canister type oil filter housing, do not trust when the Napa man say's it comes with the correct O-Ring. Order the correct pair from a dealer or contact the member out of Kentucky who makes the "Spin-on Conversion" adapter. I need to do the same thing myself and make future oil changes easier. It will still spill oil on the axle but I'll be able to source a filter much easier.
