You should talk to Johnptc,he is in the articles on this site.He has alot of information on building large tanks and he is a awsome person to talk to.I don't think you need a 2" acrylic sheet,but I am not an engineer.
On my 15,000 gallon tank ,the biggest sheet is 6'x15' and is only 1.5" thick.
Arapaimaman