I don't think it matters that the hole isn't flat bottomed.
The beauty of drilling from the outside edge is the ease of a trial and error method without the need for a full teardown.
I expect you can feel where the vibration is at its minimum and starts to come back again.
So we should be able to just drill conservatively at first, using Jaguar's calculator as a guide to drill bit diameter, and make the holes slightly shallower than the width of the crank flywheel.. Test.. Deepen.. Test.. Deepen.. Test..
Without the risk of going too far.
(Hopefully)
(Do tell me if I'm wrong, please)