I was wondering what's the best tank to use in the Demom Combat Wing that will fit properly. I will be running a Super Tigre or Fox 35.

If you have ever built the original VooDoo there was a diagram of the metal tank that was in use at the time.  Just a pickup line and overflow.  The pressure line went to the overflow if you used pressure.  I seen a lot of them flown on suction many times with Greenhead K&B, Johnson Sport or Fox Rocket.

These things were usually flown with pacifiers or bladders, as Paul said.   The tanks that people used before bladders universally took over were the Veco T-21 series.  Whenever anybody uses a metal tank, he first disassembles it to clean it out, puts the plumbing in the right places, and replaces brass tubing with copper. 

        I just recently built the Black Hawk Demon. I was able to make a bladder compartment that fit within the wing. I made mine fit between the spars and it utilized the front portion of the leading edge. Paul recommended to make the bladder compartment stick up and out of the model. I highly recommend this as well. I used a Russian .15 combat engine to power mine. This allowed for a smaller bladder which unlike yourself will be using a larger engine. My bladder tube was made out of a paper towel tube using the front portion of the ribs as end caps. I made this go under the spars which was a bad idea. I overfilled it one day and the pressure broke the glue joints on the spars. This was easily fixable but lessons learned. This was a neat plane to build due to its simplicity and mine flies pretty fast. Ken

A vienna sausage can may fit for a pacifier pod. Fill the opening with an epoxy coated balsa disk, make a hole in the side for the pacifier.

Hi Clifton
Take the measurements of the fuel compartment then go to www.brodak.com and look for the appropriate size tank, they have lots of tanks I'm sure you can find the size you need.