The way that I weld cast iron for applications like your brake drums is as follows:
Take your torch and heet up the area to be welded cherry red, almost to the point of melting. A "rose bud torch head" is also advantageous if you own one or one is avalable. Two people are better than one for this operation. It goes faster and easier if you have someone to run the torch while you do the welding, but it can be done by yourself. Just alot more work swapping back and forth. Use MG 600 welding rods 1/8 inch (a rod designed for disimilar metals) at about 180-200 amps, maybe higher. The weld will lay in like butter. Weld an inch or so. Stop and "Peen" the heck out of the weld with the pointed end of a chipping hammer, reheat and repeat. The weld will not crack.
I used to have people bring in cast items for repair from miles around. This is the method I used and I never had any complaints. Kind of expensive with the gas usage and the expensive rods, but well worth the time and effort if you have no other options and are forced to repair an irreplaceable cast item part.
I had an old time blacksmith show me how to do this. I never had any luck using nickel rod on cast. It always cracked. Finnally got with him after having no luck welding cast and never looked back. He used to repair (agricultual) combine final drives for practically pennys on the dollar compaired to thousands of dollars for replacemet. This technique may be useful for others here who incorporate brake drums or other cast items into their gasifier designs.