I've got to drive down to the garage to ask them this in person. I called them twice today and they were really busy. As I don't visit them as much as I used to for work, I think that it would be more considerate to go down there myself. When they first suggested using the oversized plug, they said that it should last for a good while. Again, it's always important not to overtighten that plug. On any older car, it's smart to always keep an eye on the fluid levels and check for leakages.
I did see an ad for threaded inserts to replace the old ones (not specifically for oil pans), but that would be a more expensive fix, plus I would think that one needs to really clean that area thoroughly for a good seal with the threaded insert... so I think retapping for a slightly larger drain plug is the simpler and better way.
And why try so hard to keep using an oem drain plug? *shrug*
On a side note... it is always important when installing an oil pan, not to over torque the bolts on that too. Only 3 ft lbs of torque is needed. Too much and it will leak. Just ask TurboMirage. B)