Thats a good design to work off of. For a 45 long you shouldn't need extra x-braces in the back. Your biggest concern will be to make absolutely sure the top is flat and level so you don't break any of the seams in your tank. It wouldn't hurt to put a nice piece of plywood on top to provide a flat level surface for the tank to sit on.
If it's a glass tank with a rim the top won't matter a hill of beans. The tank only sits on the edges. Putting a nice piece of plywood will only appease your own mind. The tank won't be resting on any part except the edges. You don't even need a top at all. Just 2 x 3 framework will take care of it just fine. If he doesn't put a skin on it I would wholeheartedly recommend X bracing to help prevent racking. 2 x 4 legs are more than adequate. Don't use pressure treated on any part of it.