Chances are your cases are ruined. Why U ask? When you weld the aluminum, the heat will distort the case in that area. Chances are also good that the drain plug is either right at or really close to the seam between the cases. Welding in this area will probably fix the drain plug, but warp the cases enough to cause them to leak now. Even if they don't leak now, they will probably be distorted enough to leak and never properly seal again if you ever take the cases apart for a rebuild.
**Disclaimer** Notice the liberal use of the words "probably" and "chances are". You may well be able to weld the cases and not have a single problem, they never leak, and everything is hunky dory.