* This broken bolt shaft is probably seized by the aluminum hole rather than by the steel threads in the block. If you could break the shaft loose from the timing cover hole, you could likely turn the threaded part out easily.
* So I agree with 2 ideas which have already been offered. First, try to heat the timing cover, around the bolt shaft, to expand the hole a little. I wouldn't try to heat the bolt shaft directly although the alum will expand more than the steel anyway. Then use whatever works to get a bite on that 1/4in.
* If heat doesn't work, then drilling would be my next choice. Needless to say, start with some new, high quality bits. Then mark the bit with a little piece of tape or such to control your drill depth. Being off center in the aluminum probably wouldn't be as bad as drilling too far and chewing up block threads.
* EZ Outs are, in my experience, anything but easy. By the time you've drilled out an 8mm shaft enough to take an EZ Out, tightening the EZ Out will expand the shaft and make it a tighter fit in the cover, exactly where it's stuck. Only a small chance of working if your seizure was in the threads. You'll either spin the EZ Out or break it off. If it breaks you're in for a good ride. The only thing (sensibly priced for the DIYer) that will remove a broken EZ Out is a stone in a high-speed grinding tool (experience with broken 3.8 head bolts and EZ Outs).

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ASE Master Tech
Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536