* Your vehicle's owner's manual: This is the most reliable source.
* The fuel filler door: The sticker on the inside of the fuel filler door *sometimes* lists the capacity.
* The vehicle's identification number (VIN): A mechanic or parts dealer can use the VIN to look up the specifications.
Without knowing the exact trim level and configuration of your 1998 Chevy 3500 van, I cannot give you a definitive answer.