About '95 or 96', GM had a real FIT with the 4.3's, making several verisons. If your's has a CRANK POSITION SENSOR in the engine's front cover, that controls the exact timing, and the distributor only has to be CLOSE to the correct postion, so the rotor is lined up with the appropriate "nipple" when the crank sensor and 'puter fire the coil. If it does NOT have the front crank sensor, AFAIK, a timing light will have to be used to finalize setting the distributor position. If it has the crank sensor up front, I believe the 'puter will output an "offset" signal to a scan tool, to verify the distributor is timed close enough for the rotor to be in the correct place when the coil "fires". I realize this post isn't of much help, however, you need to refer to the 8TH digit of the VIN, and, hopefully someone can respond specifically for the engine you have! The link below will show you how many different 4.3's there were in the 90's, although I don't think it goes very deep into the distributor timing:
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