I flew the flight tests on the big tank, I found CG to be no problem if you had a cabin load and burned the tank first (which you must anyway).
Bill Kramer and I used the aux tank to set a pair of city to city records a while back, including a Chicago to New York World Record that still stands, average speed 201 MPH for 611 NM!
(Caution - done by professionals - do not try this at home! ;-))