Here is the DuBro 16 ouncer with a Hayes four ouncer alongside.

As you can see, the DuBro barely fits.

Volfy may be right, in that you can relieve the second former somewhat without compromising airframe strength.

But if I were you, I'd take his first suggestion; fit a pump of some sort and mount the main tank BEHIND that former. There is plenty of room between the former and servos to mount a big tank transversley (sideways).

Steve