Do your L/P a favor
Use the aux tank as your main feed so the l/p will have a possitive head, then use your oem tank as storage and transfer, before you fill the aux tank or install it take it to your aluminum welder and weld in a bottom npt fitting to use as the feed line, put a ball valve under the bed for isolation . Your l/p will love you.
Read more about this by clicking on my sig.
Cheers, Kevin