Since the bolt threads into the case, you will either have to tap it for a larger bolt and drill the bolt hole on the pan to match it, or try a helicoil maybe. Never stripped a bolt on a trans. before.
Sounds like it was either cross-threaded or too much torque was used on them at re-install. Def. use a torque wrench on those bolts. The trans. is the last thing you want to have leaking.
And as said, use an OEM gasket, not the cork junk that they include w/ the new trans. filters.
These are highly engineered precision vehicles, the first step in diagnosing the problem is to strike the suspected offending part sharply and repeatedly with a blunt object, then re-test.