For a civilian Caprice, the L99 motor has a lower output alternator, 100 amps compared to the LT1's 140amp alternator. You can see the difference by looking at the alternator cooling fan. The 140amp fan has blades on the inside, like a vented brake rotor. The 100amp alternator has traditional looking fan blades. If the car is any sort of special service Caprice (9C1, 9C6, etc.) then the above does not apply. All special service Caprices got the bigger 140amp alternator.
The 8th digit VIN code for the L99 is W. The L99 was standard on both the civilian and special service Caprice. The LT1 was standard on the Caprice wagon, Impala SS, Roadmaster sedan & wagon, and the Fleetwood.
There is a machined pad on the passenger side deck surface the is not covered by the head. However, it is obscured by the alternator. That's where the codes Darth posted are located. Those codes will give you the year and displacement of the block. If the block has ever been decked, these engraved codes are usually machined off in the process.
There is also a VIN derivative located on the driver's side rear of the block, above the oil filter pad. If the engine is original, then the VIN derivative on the block will match the 3rd, and 10th through 17th digit of the car's VIN.
The L99 used two cats like the LT1, but a Y-pipe connects the driver's side to the passenger side to a single pipe with muffler and resonator.
I have been told that on the L99 block, there is 4.3 in cast raised numbers on the side of the block near the core plugs. I can't remember which side though. Kind of like 360 or 318 being cast into the driver's side of Dodge blocks. No such numbers are found on the LT1 block
I've also been told that the '96 4L60E transmission case on the L99 is two pieces, with the bell housing being bolted to the rest of it. I don't know for sure if the is just for the civilian Caprice or applies to the special service cars as well.
The best VIN locations are <UL TYPE=SQUARE><LI>On the dash visible through the windshield<LI>Federal Safety sticker on driver's door<LI>SPID label on upper left hand side of decklid[/list]
If any of these VINs do not match each other, I'd start asking some serious questions about the history of the car. Also check the underhood emissions sticker on the radiator support to see if it matches the engine codes on the SPID and the model year of the vehicle.
For LT1 Caprices the SPID should have the QMU or QMV tire codes The LT1 engine requires P235/70R-15 tires. Check tire placard on the driver's door.
The civilian L99 Caprice also has a 2.93 rear end with rear drums, code GW9. The special service L99 Caprice has a 3.23 rear end with discs on a 9C1 or drums on a 9C6, code GU5.