Much the same thing happened to me, about a dozen years ago. I had put together a replacement B20 for my 75 245, using the original head (with hardened seats) and a good 6 bolt bottom end from a 72, with new rods and mains, cam bearings too if I remember correctly. New cam and lifters. The pistons looked very good, so I put them back in, with new rings. Valves lapped in, new seals etc. Got the motor all put together, put it in the car, and was reinstalling all the accessories. For some reason, I did everything on the left side first, then moved over to the right. Manifolds on, tightening things down. It was late in the afternoon, and I was expecting to drive the car home (from my friend's shop) that evening. All went well until SNAP!!! the last stud broke on the final round of tightening. It was the one at the bottom, all the way at the back, that also holds the water pipe. No easy-outs on hand, and I don't trust them anyway (you're REALLY screwed if you break one of those off in the hole) As this was at the rear, and a 240 has a strut front end, there was no way to get a drill in there, so there was nothing for it but to pull the head back off and drill it out on the bench. I think I ended up using the drill press, which went well---I was able to get the hole perfectly centered and it went in dead straight. The first round was with a 1/8" bit, and I gradually increased the size in steps until I was at the root diameter of the threads. This left a perfect spiral of what had been the stud, that was easily removed with a dental pick, leaving the threads in the head in perfect condition. Very satisfying. Less satisfying was putting it all back together, having to wait until the next day and spend another $40 for another head gasket set (they didn't have just a head gasket at the parts store) replace ALL the manifold studs (just to be sure...) adjust the valves yet again, and so on.
In your case, I can't think of any way out other than what you did, or what I did. I agree with your theory that if you had drilled into the water jacket, it would have been obvious, with the coolant still in the engine. If I remember correctly, the oil feed to the rocker shaft is in the second pedestal, so you weren't anywhere near pressurized oil. Possibly replacing the head bolt in question seems like a good idea, as you might have nicked it with the drill. I think I would chance doing this with the engine stone cold, just pull that one out and replace it, then retorque them all to a slightly higher figure than when you originally assembled the engine. (don't go way past the factory spec, just a couple more ft.-lbs. all around)
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