The cylinder length difference is nominal, like 1/8 or 1/4". It just means it'll hold more fluid. For the larger size drum, I guess you can use the rear adjusters and extend the adjusting screws. Oh yeah and that flap for the adjusters.
Padresag has a good suggestion. Hone the cylinder and see what you got. Sometimes a kit is all you need for a repair. Pitting is usually the culprit of a bad cylinder.
If you do swap a front cylinder to the rear, you may find that the brake line fitting looks different since the front uses a hose and the rear has a junction block with a banjo bolt.
