Sadly I am :/ it was a gross overlook on my part, I have unions but they are enough to let me pull up enough to tighten because I had ANOTHER issue I had to deal with. This build has definitely convinced me to buy an AIO next time haha.I see your not fully plumed on the overflow lines coming out of the box. Couldn't you just loosen up the bulkhead for the standpipes that should give you enough play to tighten up the bulkheads going to the tank. If nothing else cut the pipe in half above the union and pull the whole bulkhead out then just add a coupling when you go to reinstall.
ETA: never mind I was looking at the picture right. Ya a dremel is likely the best bet. Good luck!