Originally Posted by Tom M'Guinn

Oil and coolant-soaked cast aluminum is a welding challenge in any environment, especially with the engine in the car, but no reason not to try -- right? The first pass or two will probably create dirty, porous welds as the contaminants are cooked out. If you grind those out, each next pass tends to get better. As for decking the surface, I'd try welding it back so the broken piece is just a few thousandths high, then use a wet-stone to "machine" it flush with the rest of the deck. If you've never ever prepped the oil mounting surface like that, as instructed by the FSM, you'd be amazed how easy it is to get a flat, glass-smooth surface across a joint like that. You are pushing a rock uphill for sure, but no reason not to try I'd say.

Originally Posted by Van

There is a reason not to try - the wasted money and time on a solution that won't work would be better spent on beer, hookers or blow...

The only way to weld an aluminum object that large is to first pre-heat it. But, even with that technique, the block will still crack (in other places) when it cools... I know from experience.