Hello,
You stated that you were building from a kit, but you didn't specify the manufacturer. Is it one of the guillow kits, or somebody elses kit? I seem to remember a 1/2A ducted fan warthog from RCM several years back. The builder used two cox td's, and cut two cox propellers down to a diameter of 3.5", then connected them together somehow, and ran the TD's with those. Nowadays, though, wemotec seems a good option. Good luck.
Kurt