Eliminating a crown is very important. For a tank over 48" long, you need full support all the way around. I had a crown on my 120 2x2x4 stand and didn't notice it, a few days after filling it I saw the seals forming bubbles and I had to tear it down & shim up the plywood on the corners.
So you have 2 options here, one is to remove the plywood and use a planer to shave the crown off of the 2x4s or 2x6s so that the plywood sits flat, or you can loosen up the screws enough to shim up the plywood and then screw it back down so it's secure and the screws are countersunk enough.
Depending on the crown, you might need very thin shims. I got a pack of cedar shakes from Menards, I think it was around $20-$30 for a pack (a large pack) and those are tapered down in varying thicknesses, some are paper thin. So you can snap them off in various widths and use 2 narrow shims inserted into the gap (one from inside, one from outside) and then you are essentially "extending" the 2x4s/2x6s such that the plywood will be level