^ Yep. Do not redrill an iron flywheel. In fact, best not to reuse them. You want preferably a billet steel flywheel.
The factory used iron flywheels because the clutch linings on the discs grip better on them then steel, i.e., less slippage.
The simplest thing to do is buy a matched flywheel/clutch set.
The iron units have been known to explode and throw shrapnel as BW stated.
Personally I'd stay away from aluminum ones as well. They are two piece and there is no guaranty that the bronze insert won't come loose, so same reasonas the iron, you could blow it up. Plus often you need to pump the clutch pedal to get the car to launch because the flywheel has lost so much inertia. That means you are slipping the clutch repeatedly which isn't a good idea.