I'm using the 20 gallon tank in mine, it is a bit taller on top, maybe 1.5" but since I put a notch in my frame I will have the space to sit the tank on the top of the frame, plus I wanted to reuse the LS1 internal fuel pump assembly.
The 16 gallon tank sit right on top and level with the frame rail of these trucks.
Look here, I have the tank inside the frame rails when the pictures were taken.
http://67-72chevytrucks.com/vboard/s...=371513&page=2
Tri5