Those dims sound incredible, badass tank man. Hopefully, you find someone to help you out.
My tank is 4 x 36 x 27. I went to HomeDepot and found a box about that size. I chose to scape it in that due to the potential mess I was going to make and it also allowed me to view what I was doing from eye level, how I'd be looking at the tank.
I used a certain type of super glue and oolite sand to bond the rock together. The joints ended up being as strong as the rock itself. It bonds within seconds too so you literally just hold it where you want, toss sand in the joint, squirt glue, toss sand, squirt glue until the joint is built up how you want it.
A tank your size, You'll probably want to create a couple structures or islands at least as a base outside the tank. Then place them in and fine tune/build off those.