Roger

all of the 1.9 water pumps that i have taken out had the housing screws
seized in it.  no mater if it was from out west or from the rust belt.  a
big dielectric reaction happening there aided by the fact that people let
their coolant go acidic.  when putting it back together anti-seize the
whole length of the bolt liberally.

something i have considered in situations like this but have never tried is
to drill out the bolt then put a small amount of diluted acid in the
drill hole and around the outside of the threads.  perhaps white vinegar
would work. this would help break the corrosion bond and therefore
lower the friction thereby actually letting a easy-out work to get the
broken bolt out.

jonathan