Despite what some Carsalesmen/Uniformed private seller tells you, the 4.3L L-99 is a V-8 and not a V-6
The Caprices/9c-1's got the 4.3L V-8 in 1994, with the 4.3L V-6's being an option in the 9c-6 (taxi)models only.
Looking at the engine under the hood, the 4.3L L-99 V-8 looks identical to the 5.7L LT-1, as stated the vin would contain a W(4.3L L-99) as opposed to a P(5.7L LT-1).
If the car is un-modified, another way to ID when looking at the rear, the LT-1 cars came with factory dual exhaust where-as the L-99's came with factory single.
Both are great dependable cars when properly maintained, the LT-1 is faster performance wise, but the L-99 isn't a slouch either...
L-99 is better when gas mileage is concerned, but again you get more power for the LT-1..
Just as a heads-up for you, the 9c-1's are likely to have their speed-limiters programmed out of their PCM's where-as a Civilian PCM with rev-limiter inplace tops out at 130-140MPH..