many old wood stands consisted of 4 longer than normal posts ( 8 to 10 feet) set and extended up above the cross brace the tank is setting on, cross brace was lower 6" or 8" inches to keep the tank from rolling off the sides, BOLT the cross bars on, and it would not hurt to cut and inset the cross brace in as well,
cross brace usually consisted of a 2x6 or 2x8.
I would suggest setting the posts 2 feet in the ground,
with the post in the ground cross bracing is not a necessity but would not hurt the build any,
some enclose the sides and make a small oil storage building under the tank,
the link has a small building and two tanks on it,
http://www.nelsonsauction.com/2009-sales/2009-06-26/100_1184.jpg
similar to the picture but out of wood