Here is what I found on w-body.com. Not sure how true this is.
8-Cylinder Engines.
The cubic-inch displacement (CID - not liters) equal the Cold Cranking Amp requirement. For example, a 350 cubic-inch displacement engine requires 350 CCA minimum. For colder climates, add 20% of the cubic-inch displacement to the CCA. Therefore, 350 x 0.2 = 70. So, 350 + 70 = 420. A 420 CCA or greater battery rating should be used.
6-Cylinder Engines.
Calculate the cubic-inch displacement per cylinder and multiply by 8. For example, a 231 cubic-inch displacement 6-cylinder engine = 38.5 cubic-inch displacement per cylinder. It is calucated as follows 231 / 6 = 38.5 cubic-inch displacement per cylinder. Then, 38.5 x 8 = 308 CCA minimum is required. For colder climates, add 20% of the adjusted cubic inch displacement to the CCA. Therefore, 308 x 0.2 = 62. So, 308 + 62 = 370. A 370 CCA battery or greater should be used.
4-Cylinder Engines.
Multiply the cubic-inch displacemet of the engine by 2. For example, a 151 cubic-inch displacement engine x 2 = 302 CCA. For colder climates, add 20% of the adjusted cubic-inch displacement to the CCA. Therefore, 302 x 0.2 = 60. So, 302 + 60 = 362. A 362 CCA rated battery or greater should be used.
Car batteries are rated by cold cranking amps (CCA rating), which indicates the battery’s power. The reserve capacity rating (RC) indicates how long you can run car accessories and still have enough power to start you engine.
Since starting a car in cold weather can take up to twice as much current to turn over a cold engine, cars in colder climates would benefit from a higher CCA rating. Check your owner’s manual for the original equipment manufacturer’s (OEM) minimum requirements needed for your car; select the battery adequate for you needs. Buying one with an excessive CCA rating may be a waste of money. In every situation, more RC is better.