It's not the weight of the edf that is the problem, it is the battery you will need. 6s batt's are not light and if you do find one of a comparable weight to the recommended 4s it will be too low a mAh rating too work.
I believe this is the combo david used in the original build and it looked like it had plenty of power. I tried building a viggen with a 64mm edf and it wouldn't get off the ground. Another instance where my cheapness got in the way of just buying what I knew worked.
Yup, the standard 70mm EDF is great for them and not too costly. I have the remains of my HobbyKing Stinger II for mine. It had a 4S fueled 64mm EDF and I'm sure it will be plenty. Heck I fly my light Viggen on a 2208/12, +/- 150W motor and it has lots of "get up and go".