If the heads aren't stripped yet I'd buy a brand new allen wrench, shove it in tight then use a crescent wrench on the long end for leverage to turn it out. If the head gets stripped out with an allen wrench you could take a dremel, carefully cut a slot on top and use a screwdriver to get them out, an old trick you've probably used before, it works great - but obviously in this instance you've got fins to deal with so it'd have to be the perfect size screwdriver, then just buy new bolts after you get them out.

Edited to add:

With the slot cut screwdriver method, if it still won't come out, I clamp the screwdriver handle laterally with vice grips and then hit the head of the screwdriver which hits down on the head of the bolt at the same moment you turn it out with the vice grips. Obviously all this takes place with the engine safe and snug in a vise.

The best way to get a stuck/stripped bolt out is to notch it on the edge with a cold chisel then give it a lateral rap with a hammer to unscrew it. But in this case you'd need a really small cold chisel to do that :-)