Would the 55's be on the "side" (if you may)? So you see the 12" deep part?
Remember, this stand would have to be either around 10 feet long+, or 4 feet deep. The 55's and 75's have the same length and width, but different depths. It would also need to be very stable (as you mentioned), because it can weigh well over 1,750lbs.
Instead of the tank being at 6' tall, I would go for 5' or 5'8". A lot of people are shorter than 6' tall, and it would be a pain doing water changes and cleaing/feeding, etc.