well you are nearly there (one side at least!)...if you can now get a drill verticle to the sheared off bolt (whether one of those flexible drill extensions might work here?), you could drill it out carefully through the centre again with a bigger and bigger drill, until the walls of the bolt are so thin, you can get the rest of the "bolt"-grooves out of the thread with a screwdriver, thus saving a re-tap?, then clean the grooves up with a tap and die. Or, drill through the centre of the bolt and put a hardened steel reverse tap into it and screw the sheared off remainder out that way? You can get hold the aluminium blocks new if you do have to destroy them, or just have them fabricated, shouldn't cost anything really.
If you are successful then put in the new bolts (preferably stainless!!) in with grease, waxoyl, anything at hand to make the removal job easier 10 years down the road!...in case.
Keep us informed how it goes, the other side ought to be easier, now you know what is ahead of you!