3800 vin C LN3 and earlier 3.8L engines had 6-bolt crank mounting flanges for the flexplate. I have no idea what flywheel someone would have to use with one of these engines if they wanted to use one of these engines with a manual transmission.
3800 Series 1 vin L L27 and SC vin 1 L67 engines have an 8 bolt crank mounting flange for the flexplate. These engines are externally balanced, so the proper amount of counter-balance weight must be present on the flexplate or flywheel for the engine to run without creating damaging vibrations.
3800 Series 2 and 3 engines are also externally balanced and have the same 8 bolt crank mounting flange for the flexplate. The N/A versions of these engines require a different amount of external counter-balance weight than the SC versions of these engines do. I don't know if any of these are the same as what a Series 1 3800 engine requires.
If you are doing a Fiero swap using a 3800 Series 2 or 3 engine and using a Fiero manual transmission, you need a flywheel from a 96-02 Camaro/Firebird 3800 5-speed application. You then need to have that flywheel machined down to 0.810-0.840" overall thickness. If you are using an N/A 3800 Series 2 or 3 engine, no further balancing to the flywheel may be required. But if you are using the SC version of one of these engines, you need to get the machined down flywheel balance matched to the spec of a stock flexplate you pulled off the SC engine.
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[This message has been edited by Darth Fiero (edited 06-26-2015).]