>I once had to put helicoils in the exhaust header holes for #1 cylinder
>with the engine in the van.  At least there was room to work on that
>one.  Of course, I have a right angle drill.
>
>What I did was cut a short length of 2X2 wood and drilled holes near
>each end (using a drill press for accuracy).  One hole was the size for
>the drill bit and the other hole was the size for the tap.  That way I
>had guides for keeping things straight.

Sounds similar to the technique a friend used recently.  She took the flange from the old exhaust system and welded a bit of tubing in the bolt hole to serve as a drill guide and then bolted the flange to the head using the other hole. The inside diameter to the tube was the correct size to drill out the bolt, leaving just the threads still in the head.  She also made some inserts to allow using this fixture with smaller drill bits.

Using left handed drills, in hopes the the bit would catch and unscrew the bolt (didn't happen), she drilled larger and larger holes until reaching the correct size.

At this point, it was possible to re-tap the hole.  I don't know if the tap just cleared the steel bolt out of the threads or cut new threads in a mix of steel and aluminum, but it worked well.  If the tapping didn't go well, the plan was to drill a larger hole and use a helicoil.

We developed this plan after I told her about my attempt to deal with a similar situation a few years ago.  I tried an ez-out on the bolt, which broke off.  I ended up replacing the head.

Bill