4 speed flywheel

Balance is as follows for small blocks.
273-318 neutral
340 steel crank neutral
340 cast crank external, different than 360
360 LA external
318 Magnum neutral
360 Magnum external, different than LA 360

As far as drilling a flywheel, if you are as much as a 1/2 a degree off on you measurements when you take the weight out, you have just throwed the entire balance of the bottom end.

McLoed makes a nice SFI flywheel that you can change weights on for any engine.