First thing to do when even thinking of attempting a repair on cast iron is trying to work out what its actually made of then you can go from there and make a repair. Magnets, Grinding spark test, weight etc are the basics.
The preheat posthead thing is kinda an old school way to do it depending on the size of the repair and all that really though. These days if the casting will play nice with a 99% nickel rod like techalloy 99 or something similar you can tig weld it in 20-25mm sections let it cool weld another bit somewhere else and so on always let it cool to the touch and never get it hot. Job done. No pissing about with a torch or an oven.
Calling something cast iron just by looking at it and hooking straight into it without some further inspection and testing is a recipe for failure on castings.
The thing with Nickel rod is its hideously expensive but its technically the correct stuff to use on cast iron. Theres also this stuff never used it sounds interesting though.
http://ezweldtigwire.com/about-ez-weld-tig-wire/