oven cleaner works great...but you'll need to wear gloves AND it tends to kill grass/plants...make sure its not the eco friendly type.
To drain the fluid, you'll either have to pull the chunk or just one of the axles and stand it upright. the axle only has a fill plug.
The e-brake cables and rubber brake line are interchangeable with the 7 1/4. Use whichever ones are in better shape. Same with the drums if the 7 1/4 has 10".
489's tend to have loose pinion bearings because of the crush sleeve getting loose over time. Check it for play.