If it's within your budget, just go with a cell. With a good external pump and a decent FPR you'll never have to worry about whether your pressure's good, or whether your pump is up to the task. I had intentions of running the factory tank initially with my build as well, but the thing turned in to a race car - and I'm thankful for it, haha. Just my two cents.
This is lookin' to be something awesome. Keep it up!