To find the exact size for your specific truck, you should:
* Check the owner's manual: This is the most reliable source.
* Look at the gas tank door: Sometimes the size is printed on a sticker there.
* Check the inside of the fuel filler door: Similar to above, there might be a sticker with the capacity.
* Check the vehicle's specifications sticker (usually in the driver's side door jamb): While it might not explicitly state the tank size, this sticker contains other information that can help you identify the correct configuration of your truck. Use this information to cross-reference online resources or a parts catalog.
Without knowing the specific configuration of your 1987 Silverado, I cannot give you a definitive answer.