You have a large footprint (8'x4') of tank, ie: nicely distributed over a large surface area.
In fact you have less weight/foot than I do with my 210.
For mine I did exactly what you described; sistering, bolting, plus added beam.
Here's my build thread if your interested:
http://www.monsterfishkeepers.com/forums/showthread.php?t=98489
As for the supports in dirt, I used 4' x 12" tubes of concrete:
Ed