You should be able to tap the existing oil pan drain hole as the land for this is pretty substantial. You said you used a bigger plug, but it didn't sound like you retapped the hole for the plug. If you did, it should be fine (don't forget a crush washer). If it leaks after retapping you may have a crack around the thread. As for leaking gas tank, these can be repaired by brazing, but you must drain, clean and vent the tank to properly prepare the surface for adherance....let a pro do this as it requires special knowledge and techniques.