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There is such a thing as "left handed" drill bits. They work by spinning counter clockwise and often will spin a broken bolt right out when they get a bite into the bolt. If that doesn't do it you can still continue with a screw extractor tool. I would only use the type that consist of a splined straight shaft that gets tapped into the the drilled hole (drill size is dictated by the size shaft needed--very important to get that right)---a hex "nut" with inside grooves to match the splines then slips over the splined shaft and that's what you use to turn the broken bolt out. Snap-On makes a kit. It's expensive but worth every penny.
I've installed dozens of Volvo water pumps on my own cars and those of clients. I use the bottom bolt hole to lever up the pump. Install the other bolts first--with the pump not flush against the block there's plenty of "slop" to allow all of them to get started--run them in to NEAR snug. Use a tapered shaft or phillips screw driver shaft inserted through the bottom pump hole and into the block---it only takes a 6-8 inch long shaft to provide enough leverage with one hand to "squash" the upper seal---tighten the other bolts evenly and when done insert the bottom bolt and run that one in. Don't use sealer on the upper "O" ring seal. I rub a paper thin layer of orange silicone sealer on the paper gasket - just enough to make it "slippery" so it doesn't bind as you raise the pump. -- Dave