I just completed a head job on a M104. What I did was buy some steel wire brushes (about $5.00 for a pack of 10) that had long wire handles. I then took a hack saw and cut the handles to the length I needed. I placed the cut-off brush in my drill and it worked very well, very fast too. I'd shy away from using a tap because if you get it off centered you'll end up cutting into your good threads, which is very easy to do with an aluminum head.

Get you some wire wheels to clean all the flat surfaces on the head, your combustion chambers and the block. You will also want to make sure that the flat surfaces are flat by taking a straight edge and placing in length wise on the bottom of the head. Then take a piece of paper and see if you can slide it under the straight edge. If the paper fits under the straight edge you will need to get the head surfaced. If not you should be OK.