New 75 gal tank build, bulkhead size question

I didn't follow any guidelines. I made mine 6"x6"x60". It stops 6"
from each end of the tank (72") to leave room for the returns. I probably could have con a little more shallow on the height. I went 6" deep because of my center bracing, it's 2' wide. I made it that deep to allow me to attach my elbows to the bulkheads first, then slip them through the back of the tank, that way I wasn't trying to thread the elbows into the bulkheads, under the center brace, inside the c2c overflow. Your going to have to just take your parts and pieces and lay it out on a piece of cardboard, the size of your tank and figure out what you want.
Matt