Drill straight down and tap to the size bolt you're using.

I have done as you have suggested, drill and tap through the head surface just outside the fire ring, screwed a piece of 3/8 allthread against the "roof" of the head, and cut and ground the allthread flush with the head surface.  Lock Tited the threads.

I have also drilled as John did, (but not the same head)but drilled the hole 1/2" and used a replacement valve guide with a 3/8 hole for a sleeve.  I drilled through the head surface, drilled and tapped the block for a 3/8-16 bolt, spot faced the inside of the deck surface for the guide to seat against, cut the guide at the top so that a 3/8 socket head capscrew would not project above the rocker stand boss, put rtv on the end of the guide and around the guide at the top to seal any water.  I now had an extra 3/8 head bolt, right in the weak area.  I later changed to 1/2" grade 8 head bolts, made sure I don't exceed 30 degrees total advance, and seem to have ended my blown head gasket woes and no longer use the extra head bolt. 

I torque the top row to 85, the bottom row to 80, thinking the top row needs a little extra because each bolt has to cover a lot more area than the bottom.  I use grade 8 bolts.  My heads are factory posted, so I don't do the added posts anymore.  3 and 6 are the only cylinders I had trouble with.  If you have problems with #3, you should also have problems with #6, since the head fits either side.  I wonder if there is a detonation or mixture problem with #3.  I have had several people tell me I should O ring it, but as we know, it is not feasible to O ring the block with the shape of the combustion chamber.  But with modern machining centers, it shouldn't be too hard to O ring the head.  Problem here is that the groove, no matter how small, weakens the already weak head surface somewhat.

There is 3/4-1" of material in the rocker bosses so there is plenty of room to thread the top. Spot face the roof of the chamber as JohnF suggests so the end of the bolt has a clean surface to push against.

All of this is done with the head sitting on a flat surface, drilling straight down. I have only done this to 2 sets of heads. Both sets were for street use but 1 set was S/C'ed and made 520hp. Haven't heard of any problems

Vic.  For the ½” head bolts on the Y, I’m torquing the short bolts to 100 lbs and the long bolts to 105 lbs.  Have experimented with up to 130 lbs but found that much bolt torque was not needed.

Are your heads posted or unposted?  I’ve seen head gasket sealing problems with unposted 'G' heads that had any serious milling performed to them in performance applications.  My rule of thumb for unposted 'G' heads is a maximum of 0.025” but I see where the Ford shop manual wants to limit the amount to 0.010”.

I have a pair of well used small letter 'G' heads that have been posted by going through the 2nd from the left rocker arm pedestal and cutting the bolt off flush with the pedestal mount.  Posting the heads this way reinforces both cylinder #3 and cylinder #6.  While this particular pair of heads were on a dominant circle track engine back in the Sixties, they were also heavily milled after the ‘posts’ were installed and held up fine with popup pistons.  I’ll try to clean up the heads this week and get an ideal exactly how much they were milled and post that information.