This happened to me before, luckily at the time the car was at my friends shop and on a lift.... Can still be done on the garage floor, but having a dealership auto shop at your disposal makes it a lot easier.

Anyway what we did was remove the manifold and find a random bolt (and matching nut) in a misc hardware bin. Set the bolt in through the top of the manifold and tack welded the head of it to the manifold. That was almost 10 years ago, been fine ever since.