Looks like your a bears fan and from Wisconsin? LOL Okay I gota help you on the bulkhead holes
Is that wood stuck to the side of the tank? Is it covering a crack or something?
Looks like you have two over flows? There is a hole going through to bottom of the tank inside of each of them, right? That will be the drain that goes into your sump. If there's more than one hole the smaller one is the return.
I saw a couple holes on the black plastic part of the overflows? They look low which is odd. Might be just the way the picture is? If the holes are at the top where the surface of the water is then those holes are the return. If they are at the bottom where the gravel would be then someone got fancy and wanted to get some of the lower water into the overflow. This is a good system to get poop off the bottom.
The holes on the bottom of the tank and on the side of the tank can only be used for a closed loop system (canister filter) and maybe a main tank drain, witch is handy when your first setting up but never used after you have fish in the tank. Most of those holes I would just put a bulkhead in and cap off never to be used again.
I'm running two canister, closed loop, systems on my 450 and I have a massive sump system. I think this is the best of both worlds and your tank is drilled for it. The canisters can clean the lower water levels and the sump/over flow system keeps the surface area clean and gives you great bio filtration and aeration. That tank will be ready for some monsters!