Well, I just successfully installed helicoils in both the exhaust
mounting holes for #1 cylinder with the engine in place.

I cut a couple of pieces of 2X2 wood and drilled them for the drill bit
   and for the tap.  placing them flat against the exhaust port, they
worked as a drill guide and a tap guide.  Of course, I had to use a
right angle drill chuck attachment.

I just can't imagine trying to do a good job without some sort of drill
guide.

Mike
'85 GL Westy (and others)