I just had the same situation. Luckily my wife works at a construction company and was able to get me wooden shims that were about 18' long by 4" wide and were tapered from about 3/8 of an inch down to 1/32.
I called All Glass Aquarium ,who makes my tank (46g bow), and they told me that with my type of stand (pine) I should only put shims on the sides (front corners) since the front/middle of the stand is not really loadbearing.
I found that when I was leveling the tank I was getting different readings depending on where I had the level on the tank (Top, bottom of top trim, top of bottom trim, stand) I ended up using the top of the stand as my final reference.
It's been set up for a month now and still looks good. If you have the time you might want to shim the stand, fill with water, and see if it settles at all before you cut the shims to length.
Good luck!