And what would happen to the five fish already in your tank? Surely not into the 55 with them?
When will you be getting this custom tank? If it's soon, why not just wait for the big tank rather than rehouse five fish for two new ones who may not even get along with each other in such close quarters? Silver Arowana grow fast, you'll be surprised at how soon your 280 gallons become obsolete.
Doesn't seem like a good investment, considering the large tank you'll need to house all your other fish while your Arowana is in the 280.
We need more information, but thanks for flying off the handle like that. Much appreciated by people trying to help you.