Let me get this right, you don't know if the rear axle is five lug or six lug based on the brake drums??

All Waggies are six lug. The axle you bought is the same width as the one that would have accompanied the front axle you bought, so the match is a good one.

As far as messing up the perch on the pumpkin, grind a little and bust out the angle finder. If you level the old perch and grind the new perch so that it is parallel with the old one, you're golden. Just use a small bubble level to check the angle. Grind slowly, measure often. Patience is the key to prevent a mistake.