I had to completely drain my tank because of a bad fuel load. When I filled it back up (the 1st 5 gallons were from a can, drove 5 miles and filled it up the rest of the way) I put ~37 gallons into it - plus about a 1/2 gallon of additives. I did fill it ALL the way to the top of the neck (so that I treated the fill tube as well) and as I understand it they usually have the tank set up so that there's 2-3 gallons worth of air space for fuel tank crush in an accident - so in my case my fuel tank holds about 35 gallons. There is a TSB out there for a red cap on the rollover valve on the top of the tank that should be removed (on some trucks it wasn't evidently, not sure if that applies to your 96 or not). If that cap isn't removed the tank will crush under the vacuum and you'll lose some capacity.