I get the cast iron rod from Tucker Valve
Seat co in texas. The best flux is from Superior
flux in cleveland oh.
Break a piece out of an old bath tub to make
the repair piece , very good cast iron. Do heat
the whole manifold to at least 1000 deg before
you weld, and heat 1500 and hold for 15 minutes
and very slow cool from 1500 to 1200 ( 30
minutes) then slow cool on down . The rate from
1500 to 1200 will make the cast very machinable,
if it chills in that temp range ,it will be
very hard and even carbide tooling doesn't
like it . high speed tooling won't touch it.
Below is a before and during repair of a
wauk manifold. Also a link to more of that