You're going to flower 72 plants in a 4x4 (3x3 tray)? How are you going to keep them that small? Have you found mini-mj plants? LOL. I hope you've done extensive reasearch on the techniques of "Lolipopping". If not, you'll be in serious trouble.
Everyone has a plan ... it's not always a good plan though. My tent is 2x your size. I'm flowering 8 plants right now, and it's fuggn stuffed in there. A common misconception is that more plants equals more yield. NOT! Plants packed together will fight for space and race toward the light.
I will save you thousands of dollars and many headaches. Start reading dozens of grow journals. Pay attention to the size of the plants when they finish. Watch the successful grows and the unsuccessful ones too. The threads that abrubtly end were most likely failures. Find threads that grow plants exactly the way you want yours to come out. Rinse and repeat.