Busted a bolt on the cowl vent

[rally]

I got 3 of the 4 out, but the last one I busted the head off the bolt (I can't remember the size off the top of my head, but want to say it was maybe a 5/8). Anybody have any tricks to getting it out?

I've soaked it in PB Blaster for a few days and tapped it lightly with a hammer, hoping to break some of the rust before I start, but I don't have an EZ out set or a way to cut a groove in the top of the shaft to spin it out with a screwdriver. It's probably too rusted to spin it out anyway. It's pretty much sitting flush, so I can't take a cutting wheel to cut the groove or grab it with vice grips, either. Was thinking maybe a chisel?

But I wanted to check with you guys to see if any of you had some cool tricks. I don't have the EZ Out set or a chisel set, so whatever route I take I'm going to have to buy the tools. So that leaves me open to suggestions.