This back bolt rusts/corrodes out all the time. Tapping in the cast iron is Ok, just take it a bit at a time, and tap it dry.
On mine the back bolt was all corroded off at the exhaust manifold, so IIRC, I carefully drilled out remaining bolt part in the ex manifold and installed an insert' I stayed with 1/4" I think and just used a 1/4" bolt cut and threaded to fit, and slathered it with anti-sieze. See below as to why that back bolt diameter does not matter all that much in the field.
I only snugged these 3 bolts down until the manifolds were snugged pretty well up to the heads so that the manifolds could float a bit as the head studs were torque down; that is the way it should be done anyhow. Watch the torque on those bolts; it is not all that high, like maybe 8 or 10 ft lbs.
