Well if you already have a new tank to replace the cracked one, just get some wood shims from Home Depot or Lowes and shim the stand before you put the new tank on it. It doesn't sound like you will need much to level the stand.

As far as the cracked tank, in my opinion without actually seeing the tank, it doesn't sound like your stand was that unlevel to cause a tank to crack like that. BUT, as your tank had been redone/resealed, it could have been possible. I'm not sure if they have improved in the quality of their work or not lately, but if you do a search you will find Forever Pets has a bad history when it comes to resealing tanks. There have been several complaints of cracked tanks and poor customer service after the fact.