If the heads are off measure the bore, a 305 is 3.736" and a 350 is 4", they both use a 3.48" stroke crank but a 305 crank is about 50 grams lighter than a 350 crank.
If the engine is 1986 and newer it will be cast into the driver side bell housing with either 5.0 (305) or 5.7 (350)
If still running the factory-equipped engine, you can tell by the 8th letter of the VIN. 'E' = 305, '7' = 350. Also, on the SPID label on the trunk will be 'L03' if it's a 305 or 'L05' if 350. If the engine has been swapped, you'll have to check the bellhousing, which is difficult to do with the engine in the car, but can be done with an inspection mirror.
On Caprice sedans, only 9C1 police cars or cars with the LTZ package had 350's available from the factory.
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