Jon,
short answer is no, you cannot weld to the block. The long answer is that cast iron can be welded using special rods, maybe special welders, lots of skill and experience, and probably some way to preheat the area to be welded. But for us amateus, just no.
There is some risk using any electrical welder on computer controlled cars. I don't know how big the risk is, but I've seen it mentioned in other places. There may be ways of reducing the risk, maybe by disconnecting the battery terminals, but I'm not really sure.
Last, I'm not sure if you want to weld brackets to the car, but an arc welder isn't the best way to do that. It would be very easy to simply blow big holes in the sheet metal and not get anywhere.
And one more thought: pay attention to what's on the other side of where you're welding. It's easy to forget about insulation and carpets, but it only takes a couple of seconds of welding to set them on fire...
There are tons of things to know about welding before you strike an arc on your car, and in particular you need to know the limitations. Some types of welders can weld upside down, some can't; some will weld sheet metal, some won't. Find a good book about welding and read everything that applies to arc welding, then practice a lot on scrap metal of varying thicknesses to see what happens.
Good luck!
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