Just banging around the internet and I knew/know some guys who flew the Demon which as you may know, was like a lot of the 1950 era fighters.. very limited by its engine and the engine proposed did not come up to expectations. Also, an engine replacement required almost dismantling the airplane, not exactly user-friendly. When the J40 powered Demons came off the line, they performed SO BADLY that many were loaded on barges, shipped to maint training bases and NEVER flown. McDonnell changed to the J-71 engine, still not the best solution. Still not what McDonnell or the Navy wanted, came up with the F3H-3G or Super Demon F4H. There were different models, some single seat, some two seats. Proposals also included two J65 engines or the monster J79s. That is the picture you see and in some ways, they are correct in that in it was part of the evolution. Note that after wind tunnel testing, aero improvements included the dihedral and dogtooth wings as well as the horiz slabs with anhedral. It was, to say the least, a work in progress.

FWIW, McDonnell planned to call the airplane "Satan" which for some reason didn't impress a lot of people in DOD or Navy.

One site:
Welcome To the Warbirds Resource Group - Vietnam Conflict Aviation Resource Center