drilled

Good morning all, I'll currently in the process of setting up a drilled 270 (84x24x31) and as much as I'd love to do a sump the weight constraints of the tank location won't allow me to put another 1200 pounds of weight on floor. Long story short this will be the forever home for my Fahaka...

I have a 240G. Acrylic with drilled drain and returns in both of the back corners. My question is, when I hook up the drain and return piping. Do I want the rubber seal on the inside of the tank or outside? I have all the bulkheads, rubber seal, tubing and whatever else.... I just dont know...