[Bruce Lancaster]

Retapped holes are in no way a killer, as most blocks have already been rendered to class 2 threads there by someone who thought he was doing a good thing. Sealing and stud alignment can be fixed, the strength lost cannot be but what remains is adequate.
No, you do not want the flat bottom taps either...and they wouldn't change anything since the holes go right through.
The original holes were tapped with taps of different thread class, meaning deeper/taller thread than normal type two, much firmer engagement. Tight classed theads cannot be fully assembled by hand...if you can spin them in by hand, they have been cut back to class 2. Most things in cars and in general hardware are class 2, the higher classes are for applications like this where maximum grip and /or sealing are required. Do not tap them unless threads are actually damaged. Clean with metal gun brushes and spray carb cleaner, or something like a '49 headbolt prepared for the job: Notch it like a tap so it cuts away crud within threads, before using run it into a die and heavily wire brush it to make sure your notching did not leave any raised shards that would cut away metal in block.