broken exhaust studs

The manifold to pipe studs ought to be drilled out and new bolts put in as said above. Buy some BRASS nuts and hardened SAE washers to go with the grade 8 bolts and they will be sooooo much easier to remove in the future. Brass nuts don't corrode to steel bolts, and the SAE washers are less wide and fit better.

The holes where the long inter-manifold bolts went need to be carefully drilled out (carefully so that the centers stay where they were) and heli-coils installed. The ones that break off are so corroded in place that there typically can't extract anything.