I wish I would have seen this earlier.
I've done what you did more than once. The factory service manual for my '68 Charger calls for something horrendous on rocker shaft bolts, too.
Be sure you vacuum the daylights out of where you drilled. You are going to want to heli-coil it.
Also, most of those holes are solid, but I will warn you that one of them has an oil passage that feeds the rockers on it. If you took the shaft off, make certain that the orientation of the oil holes are correct and it is put back the same as it was removed, or you can have an oil starvation problem.
Which of the five bolts broke and on which cylinder bank? Both heads are the same, but it would be a good idea to let everyone here know, to determine that you don't have drill shavings going down an oil passage.
I don't torque the shafts. I can't stress that enough. I go hand tight on a typical 3/8" ratchet and follow the sequence and I have never broken anything since.
Typically, rocker shafts do not come loose, even under odd stress. You will see bent pushrods, stuck lifters and even trashed ones along with flattened cams, before you see a rocker shaft come loose.
If you are looking for a tic, pull both valve covers and remove the rocker shafts. keep the rockers on each shaft and just set them in their respective valve cover. It's best to leave the bolts and spacers under the bolts in their holes and just set them in the valve cover for that side.
Once you have the shafts and rockers off, lift one pushrod out at a time (you will feel a little vacuum from the oil in the lifter) and it will release by hand. Roll each pushrod on a flat glass surface and check each one for straightness. Replace the bent ones ($2 ea) and put the pushrods back into the engine. When you do, put oil on each one and when you feel like they are back in, spin it in place and it should feel nice and snug, like a pool cue in chalk. If not, grab a flashlight and look down the hole to see if it's sitting centered in the index.
Rocker shafts are way easier to put back on if you pre-assemble the shafts with bolts and spacers. If you hold them by the sides and use the valves/retainers to rotate the rocker arms into place, going back on, from how they want to hang upsideown, they rotate into place easily and the bolts drop into the rocker shaft pedistols.
If you find a bent pushrod, don't stop and only replace one. check all of them.
rayer:
