Out the bottom. Remove the filler neck and the three mounting bolts and fuel line, then slide it toward the drivers side and drop the right side down. Probably have to take the exhaust loose, and may have to pry a little to get it far enough to the left to drop it down.
Never removed one on a 39, and filler necks vary. But they all (35 - 40) should come out the bottom, since the frame is the same and tanks only significant differences were filler neck related.