My wife isn't either Richard, which is why I come here for it! She's more into tennis which is not my dish of tea. I've learnt more about tennis players over the decades than she has about camshafts, though, I have to say!
Hadn't realised the cases were back in the frame - that is a bore. However, with care, loads of clean and as they used to say 'lint-free' rag round the opening and the rods, and a very careful approach to drilling and tapping, I reckon you'll be OK. The toughest thing to ensure is 'verticality' if there is such a word. To try to ensure that, I have been known to make silly little bits of metal to bridge from A to B or a triangle even (whatever I can get a good fix on) directly above the hole to be dealt with, and use a precisely drilled hole in the bridge piece as a guide for drill, tap, whatever. A bit of a palaver to get the hole in the right place, and requires some mental exercise, but it can help a lot. To get your hole straight and where it's needed I'd put a good-fitting drill or bit of steel in the hole to be redone, check it's vertical with a T square or something else with a true rt angle on it, assemble the bridge above it, put something sticky on top of the bit of steel/drill, raise it till it touches the bridge and leaves a circle of yuk, withdraw bridge, drill hole, reassemble bridge, etc etc . . . There's nothing worse than crooked holes, as the nuts don't go down right on crooked studs. Friend of mine did a wrecked Velo Venom crankcase-to-cylinder thread this weekend with the motor in the frame - no hassle. Started mid-am and was out and about on it mid afternoon. So no worries, as they say down under. And you are definitely doing the right thing in my opinion.