I just want to make sure I'm understanding you correctly: You want to do 8,000 GPH through a single, gravity fed pipe, on a tank? What is the 3 x 2" regarding?
For 8,000 GPH, I don’t think even a 2” siphon will be sufficient. More like 3”. Assuming you’d want the bean emergency to be able to handle 8,000 GPH in a, well, emergency, I think you’d need 4”.